The names of colors are not proper nouns. Color names are not capitalized unless they appear as the first word of a sentence.

Do Colours have capital letters?

Nope, a color is a color. You don't capitalize poppy red do you? They are not proper nouns, therefore you don't need to capitalize them.

Are colors considered nouns?

As detailed above, 'color' can be a noun, an adjective or a verb. ... Noun usage: Most languages have names for the colors black, white, red, and green. Noun usage: He referred to the white flag as one "drained of all color".