​Does color exist in the world external to our minds?

First off we have to accept that there is an external world – a world external to our own experience of our mind.

Getting past that one, we have to accept that there we are one of the objects that exist in this external world and that we interact with other objects in that same world.  We are a mind/body unit interacting within this vast universe where we live on a planet that we call the Earth.

We perceive objects that have certain characteristics – properties that we associate with those objects that we experience.  We can pick up an object, such as an apple and we can describe that object by means of describing its shape, weight, and texture.  We can share this object with a companion and come to an agreement about these properties that we agree the apple has.
Now, what about the property of color?  Is the apple green in the same way that it has a noticeable smooth texture when we feel the outer skin of that apple?

There has been a long-standing agreement that all objects have two broad categories of properties, so-called primary properties which are ‘objective’ as in adhering to the object, and secondary properties which are ‘subjective’ and thus more a product of our mind.
 
 
From Wikipedia: https://en.wikipedia.org/wiki/Primary/secondary_quality_distinction
 
The primary/secondary quality distinction is a conceptual distinction in epistemology and metaphysics, concerning the nature of reality. It is most explicitly articulated by John Locke in his Essay concerning Human Understanding, but earlier thinkers such as Galileo and Descartes made similar distinctions.
 
Primary qualities are thought to be properties of objects that are independent of any observer, such as solidity, extension, motion, number, and figure. These characteristics convey facts. They exist in the thing itself, can be determined with certainty, and do not rely on subjective judgments. For example, if an object is spherical, no one can reasonably argue that it is triangular. Primary qualities as mentioned earlier, exist outside of the observer. They inhere to an object in such a way that if the object was changed, i.e. divided, the primary qualities would remain. When dividing an object, “solidity, extension, figure, and mobility” would not be altered because the primary qualities are built into the object itself. Another key component of primary qualities is that they create ideas in our minds through experience; they represent the actual object. Because of this, primary qualities such as size, weight, solidity, motion, and so forth can all be measured in some form. Using an apple as an example, the shape and size can actually be measured and produce the idea in our minds of what the object is. A clear distinction to make is that qualities do not exist in the mind, rather they produce ideas in our minds and exist within the objects. In the case of primary qualities, they exist inside the actual body/substance and create an idea in our mind that resembles the object.
 
Secondary qualities are thought to be properties that produce sensations in observers, such as color, taste, smell, and sound. They can be described as the effect things have on certain people. Secondary qualities use the power of reflection in order to be perceived by our minds. These qualities “would ordinarily be said to be only a power in rather than a quality of the object”.  They are sensible qualities that produce different ideas in our mind from the actual object. Going back to the example of the aforementioned apple, something such as the redness of the apple does not produce an image of the object itself, but rather the idea of red. Secondary qualities are used to classify similar ideas produced by an object. That is why when we see something “red” it is only “red” in our minds because they produce the same idea as another object. So, going back to the color of the apple, it produces an idea of red, which we classify and identify with other red ideas. Again, secondary qualities do not exist inside the mind; they are simply the powers that allow us to sense a certain object and thus ‘reflect’ and classify similar ideas.
 
According to the theory, primary qualities are measurable aspects of physical reality; secondary qualities are subjective.

• By convention there are sweet and bitter, hot and cold, by convention there is color; but in truth there are atoms and the void"

—Democritus, Fragment 9.[5]

• "I think that tastes, odors, colors, and so on are no more than mere names so far as the object in which we locate them are concerned, and that they reside in consciousness. Hence if the living creature were removed, all these qualities would be wiped away and annihilated"

—Galileo Galilei, The Assayer (published 1623).[6]

• "[I]t must certainly be concluded regarding those things which, in external objects, we call by the names of light, color, odor, taste, sound, heat, cold, and of other tactile qualities, [...]; that we are not aware of their being anything other than various arrangements of the size, figure, and motions of the parts of these objects which make it possible for our nerves to move in various ways, and to excite in our soul all the various feelings which they produce there."

—René Descartes, Principles of Philosophy (published 1644/1647).[7]

• "For the rays, to speak properly, are not coloured. In them there is nothing else than a certain power and disposition to stir up a sensation of this or that colour."

—Isaac Newton, Optics (3rd ed. 1721, original in 1704).[8]
 
Now, I think that this split between primary and secondary is not as important or accurate as they seem to think.  I propose that all those properties are part of the object and our interaction with that object.

Let’s take color.  It is the one most picked on by philosophers and scientists as being the most subjective – not a ‘truly real’ property of any object.  Color is created by light hitting an object and then coming at our eye at a certain wavelength which is registered in our mind as being of a certain color.  The light hitting an apple we end up seeing as reddish or greenish, depending on the apple we are looking at.

Now, if Eve holds out an apple to Adam, they can both come to an agreement on what color to call that apple.  Thus there is persistence and consistency between multiple observers and the object being observed.  That is what all the characteristics of that apple have – its shape, texture and even its color are an agreed term for what we experience when we examine objects.  The apple has something that gives rise to that agreement of description.  Thus the apple ‘has’ those properties, whether they are shape, texture, and even color.

Neither the shape, texture nor the color fluctuates from moment to moment.  It is agreeably stable and both Eve and Adam together can contest to those descriptive terms as being reliably useful as a means to describe that apple that they are sharing together.

Our bodies interact with that object and by the means of the interaction, we come to describe and know that action.  All that interaction is ‘objective’ in that it is derived from our interaction with the object.

Color is as real as taste and any other property.  It is a shared property of objects and observers like any other.  Any artist knows that.  She works hard to mix just the right pigments to get exactly the color she desires.

As William James would remark color has a pragmatic and literal cash value in our world.

Color is real and it is the stuff of important legal and business decisions.  The whole of the entertainment and advertising industries depend on objects having the proper and consistent colors needed for their businesses.  People are paid cash to get those colors right and lawyers can sue if printers or manufacturers fail to get those colors to match to the legal trademarked Pantone color that the corporations require!  So all those businesses understand that color is real and part of things that have to be manipulated just right in the world to meet their needs and requirements.  They understand that color is a real thing, a real attribute of objects.

Pantone LLC is a limited liability company headquartered in Carlstadt, New Jersey.  The company is best known for its Pantone Matching System (PMS), a proprietary color space used in a variety of industries, notably graphic design, fashion design, product design, printing and manufacturing and supporting the management of color from design to production, in physical and digital formats, among coated and uncoated materials, cotton, polyester, nylon and plastics. 
Pantone began in New Jersey in the 1950s as the commercial printing company of brothers Mervin and Jesse Levine, M & J Levine Advertising.  In 1956, its founders, both advertising executives, hired recent Hofstra University graduate Lawrence Herbert as a part-time employee. Herbert used his chemistry knowledge to systematize and simplify the company's stock of pigments and production of colored inks; by 1962, Herbert was running the ink and printing division at a profit, while the commercial-display division was US$50,000 in debt; he subsequently purchased the company's technological assets from the Levine Brothers for US$50,000 (equivalent to $430,000 in 2020) and renamed them "Pantone".
The idea behind the PMS is to allow designers to "color match" specific colors when a design enters the production stage, regardless of the equipment used to produce the color. This system has been widely adopted by graphic designers and reproduction and printing houses. Pantone recommends that PMS Color Guides be purchased annually, as their inks become yellowish over time.[7] Color variance also occurs within editions based on the paper stock used (coated, matte or uncoated), while interedition color variance occurs when there are changes to the specific paper stock used.[8]
Pantone colors are described by their allocated number (typically referred to as, for example, "PMS 130"). PMS colors are almost always used in branding and have even found their way into government legislation and military standards (to describe the colors of flags and seals).[13] In January 2003, the Scottish Parliament debated a petition (reference PE512) to refer to the blue in the Scottish flag as "Pantone 300". Countries such as Canada and South Korea and organizations such as the FIA have also chosen to refer to specific Pantone colors to use when producing flags. US states including Texas have set legislated PMS colors of their flags.[14]
(https://en.wikipedia.org/wiki/Pantone )

Now it is true that an artist can adjust the amount of light or even use filters over the light source or different light bulbs all in order to alter how one observes the object in those altered lights.  But note that the artist can deliberately and in a controlled manner take these actions to yield a result that the artist chooses.  Now, altering the illumination on an object doesn’t change the object's shape or weight, hence why the classification schemes of primary and secondary were made up.  We cannot alter an object's shape, weight, and those other so-called primary characteristics.  So there is something different about the shape of an object and its color.  However, there are still some attributes that are inherent in the make-up of an object that the illumination that impinges on it and is the means for the observe to see the object can be manipulated in a controlled and repeatable manner yield similar end results.  Thus, the object has to have a fixed property that can so be perceived consistently on repeated specific illuminations.  Thus, the object has to have those properties as part of its make.  It is not merely in our minds.
​
If color was not a property that could be controlled and made according to chosen specifications then so many industries would have ceased to exist.  Therefore it is foolish to claim that color is a purely subjective property.  All sorts of objects are made to have a certain color and thus it should be recognized that color is as real an attribute of some object as its shape.  Objects have properties that give rise to consistent and agreeable descriptions between observers that is the very definition of an object's attributes or properties.