[thaumasiotes]

I can provide some non-native commentary on modern Mandarin Chinese. All color terms are most ordinarily used with 色 ("color"); it is unusual, though possible, to use a color term by itself.

It doesn't seem like a big stretch to imagine that this was also true when the terminology was borrowed into Japan from China, and that's why the 色 gets used in Japanese.

[howlgarnish]

I can't claim to be an expert, but this is likely correct. The native Japanese words for color (shiro, kuro, aka, ao, midori etc) don't use the -色/iro suffix.

[naniwaduni]

Japanese has the cool feature that you can clearly identify siro/kuro/aka/ao (a fairly typical four-way white/black/red-yellow/blue-green distinction) as the most basic colour words with relatively little diachronic analysis: they're the ones that stand alone in the rather limited class of 形容詞 ("i-adjective") stems.

You can almost trace the development of colour terms through the vocabulary strata...

[EE84M3i]

Note that you still can use the the iro/color suffix for aka (red), ao (blue), midori (green), you just don't have to and they also function as standalone adjectives.

E.g. akai (赤い) -- red (adjective) vs akairo (赤色) -- the color red (noun)

compare: chaiiroi (茶色い) -- light brown (adjective) vs chaiiro (茶色) the color light brown (noun) -- literally "tea color"