[mongol]

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).

[mdp2021]

Sure, but those colours still «are electromagnetic frequencies». The poster stated that some colours are not.

[mikelevins]

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".

[mdp2021]

> If the colors were the same thing as

Oh, nobody said they overlap.

The original post claimed that «Colors don't exist outside of your brain»: such statement, while true, is false, as its negation is true (we have mentioned paraconsistency in this very page): "colours" do exist outside your brain, as their nature also is, in a way, being electromagnetic phenomena, function of frequencies.

While such ontological property is pretty common, it is just summoned with some force when someone claims "[fuzzy] is [strict]".

> Thus, electromagnetic spectrum is "out there", but color is "in here"

Of course. That just depends on what you want colour to be. If you put it conceptually near "electromagnetic spectrum", then the distinction emerges.