Colours are, in the outside world, fuzzy ranges of electromagnetic frequencies: they do «exist outside of your brain».
(Similarly, "brain" and "mind" are not the same.)
Otherwise, the intended general idea is that found e.g. at the beginning of Arthur Schopenhauer's Welt als Wille und Vorstellung:
> "The world is my representation": this holds true for every living, cognitive being, although only a human being can bring it to abstract, reflective consciousness: and if he actually does so he has become philosophically sound
Some electromagnetic frequencies are colors, but not all colors are electromagnetic frequencies. That’s why parent stated that color is a brain phenomenon.
What do you mean? (Outside the detail that "hues" are electromagnetic frequencies, while colours are compositions - there I just simplified.) Which colour is not such?
I would explain it as colors are byproducts of electromagnetic frequencies, but they are qualia generated by your brain. There are many optical illusions that play with this fact. For example, in twilight, the frequencies you would call blue are different than what you call blue during daytime. This is because the brain /eye adjusts to the general light conditions. (As sunlight is generally "redder" at twilight).
If the colors were the same thing as the electromagnetic frequencies, then the same electromagnetic frequencies would be the same colors, by definition.
They aren't. For example, put a card of color A in front of a background of color B; now move it in front of a different background of color C. You will experience color A as being a different color (especially if colors A, B, and C are chosen to maximze the effect).
The electromagnetic spectrum returned by card A isn't different, but the color perceived is. Thus, electromagnetic spectrum is "out there", but color is "in here".
It is the sum of two ranges (magenta is the sum of red and blue lights): it also exists "outside", like the rest.
For that matter, not even "pinkish grey" is defined by simply a frequency (the hue is, the colour is not): the definition for this purpose was meant to be concise, not literal.
Your previous comment had stated that hues are electromagnetic frequency and parent comment showed a counterexample. Color is a complex phenomenon that cannot be reduced to electromagnetic frequencies and their composition. For an example, see impossible colors [0].
The distinction between colors exists outside our brains (different frequencies of em radiation), but the color "red", for example, only exists in your brain. Heck, I have no clue what someone else perceives when I perceive "red". As long as it is stable, it doesn't matter.
Well, our brains are only wired up to detect a limited spectrum of colors. There are insects and birds that can detect a greater variety of color. So, how can it just exist in our brains?
One way to look at colors is to see them as qualia, which is basically a subjective experience. Electromagnetic waves come in a large range of frequencies, some of which excite the nerve endings in your retina, causing your brain to experience colors. But colors are a bit more complicated than that: for example, there is no such thing as just "yellow": yellow is the subjective term given by humans when there is a certain balance between the excitation of their there-color vision. You can experience the exact same sense of "yellow" from different types of light: there is true monochromatic yellow around 580nm, but you can also experience that same yellow when mixing the right amount of red and green light. There is no wave of 580nm in the second case, but still you see the same color.
Our eyes are wired up to detect a limited spectrum of wavelengths. Although those wavelengths correspond to a color, not all colors correspond to a wavelength. That’s why OP stated that color is a brain phenomenon.
(Similarly, "brain" and "mind" are not the same.)
Otherwise, the intended general idea is that found e.g. at the beginning of Arthur Schopenhauer's Welt als Wille und Vorstellung:
> "The world is my representation": this holds true for every living, cognitive being, although only a human being can bring it to abstract, reflective consciousness: and if he actually does so he has become philosophically sound