Person asked:

How many colours are there?

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An interesting question. I'll ask you one: what is "a color"? We can say, "I see red" when we look at a
red light; but we're not seeing red, really. After all, "red" is a class-name, for a large set of specific
colors, right? We are seeing an example, an element, of that class. So what we're doing is seeing a
red light, with a very particular exemplar, a token, of the type: red. Ok, fine. So then that particular
token is, let us say, "a red". But if we say that, then as you can see there are an infinite number of
colors, right?

But let's say that you are asking something like, how many types of colors are there? Now, I could
say, with complete seriousness, that there are no colors or types of colors at all, any more than there
are numbers "out there" in the world. We construct colors from our processing of wavelengths of
radiation, just as we construct numbers by abstracting from the operation of counting. So in one
sense of "are", there are no colors at all.

But hey, let's get past that, and take this fairly casually... how many colors are there. Ok, there are
three. Well, four, if you count the rods in the eye. We have receptors for (wavelengths corresponding
to) red, blue, and yellow. We combine those and make things like orange and purple, but those are
complexes of the three primary colors we have receptors for (and in dim light we use the rods which
are blue-sensitive as well, but I'm not actually sure that they combine in the same way as the others).
So that's another possible answer.

On the other hand, further up in the cortex we do opponent-processing, where red-blue pairs interact
with, I think, yellow-green (?)... I'd have to look it up. At any rate there are three of those pairs also.
So maybe we see three primary colors and three secondary pair-colors. But then I don't know how
many colors there are, because what's a "pair-color"? But that's how we do it. I guess you could say
that there are nine colors in that case, perhaps.

But on the other hand, an artist would say I was being an absolute idiot, and that there were millions
and millions, maybe an infinite number, of colors... because we can discriminate all sorts of shades of
colors. I actually do not know whether anyone has attempted something like changing a color mix by
one photon, or by the smallest fraction of a wavelength they could generate in a lab (which is actually
quite small) to see if people could see that difference. That would be the test, I guess. Then you'd just
multiply, or so you'd think. But the problem with that is that the human visual system doesn't work that
way. Sensitivity is dependent on context: the setting you're in; your expectations; what you've last
seen; how bright or dim it is; your color sensitivity is extremely variable, just like all your other senses.
There are circumstances in which you can see single photons, if the light is dim enough... but you'd
never be able to do that in bright light. So... how many colors are there? It depends on where you're
seeing them.

Steven Ravett Brown

In English there are eleven colours: Black, white, red, yellow, blue, green, brown, purple, pink, orange
and grey. According to George Lakoff (Women, Fire and Dangerous Things) these are the 'focal' or
purest categorisations of colours. Sometimes a colour term is contained within the focal category, as
scarlet is contained in red.

The numbers of colours identified in a language can vary greatly between languages. However, there
are six primary colours black, white, red, yellow, blue and green which are psychologically real even if
a people does not name them.

Rachel Browne

How many colours are there?

None whatever. The whole thing is a big mistake. Just ask any bat.

(PS: Someone just recently asked a similar question. So let me refer you to my answer there. Just
look for the word "colour".)

Jürgen Lawrenz

Sydney