The statement wavelength is defined by its color is not correct, and is needlessly solipsistic. Color is the result of the wavelength as interpreted by the eye, but the condition that allows color to be interpreted to begin with is independent of the eye.
Without the brain, colour would not exist, but wavelength would.
It's the old... do you hear the sound of a tree fall if no one is around to hear it.
Would you see the color of red paint, if no human were around.
I think the color red does exist, even if it exists only in our mind, it exists.
You and i both agree, we see red, and our reality of red is the color we see.
And we both agree, it exists, only in the head.
Would you like to know what else only exists in our heads?
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Justice, love, morality, mercy, freedom and a plethera of other emotions and ideas.
So don't think, if it exists in our head, but isn't tangible outside it, that it doesn't exist.
I can quite easily say, all those emotions exist, even if some of them are the result of chemical reactions.
I can't say though, morality, is the result of chemical reactions.
Morality would actually be, you working against your chemical reactions.
Against your inner evils, inherent in all humans, rising above them, through your ethics and morals.
So does color exist? Yes, in my opinion it does, just like justice, love, morality, and a million other emotions and ideas only exist in our heads.
~~Datruth