[howlgarnish]

In many languages the distinction is quite blurred. In Japanese, most color words like 茶色 chairo (brown), 灰色 hairo (gray), 黄色 kiiro (yellow), 紅色 beniiro (crimson/red) incorporate the word "iro" (color), but while the first two are tea-color and ash-color, ki 黄 and beni 紅 now just mean "yellow" and "red" respectively.

[schoen]

It looks like 黄 and 紅 never had a distinctive non-color meaning (as characters) without 色, so maybe using them without 色 is more a recovery of their historic meanings rather than an invention of a new meaning?

(That's just a guess, I especially don't know the roles that written and spoken forms have played in the evolution of these meanings, or whether spoken "ki" and "beni" did or didn't have independent meanings.)

[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.