vocabulary - Historical differences between colors that are i-adjectives and those that are simply nouns

EDIT: Started a bounty with hope of getting more definitive and elaborate answers, e.g. timeline of when color names started being used in Japan.

In Japanese language, there are colors that are i-adjectives: 青い, 赤い, 黒い, 白い etc.

Then there are colors that are original nouns: 緑 {みどり}, 紫 {むらさき} etc.

And finally color nouns that are made of [something]-色: 茶色 {ちゃいろ}, 黄色 {きいろ} , 灰色 {はいいろ}, 桃色 {ももいろ}, 銀色 {ぎんいろ} etc.

Why are there different word classes for colors in Japanese? Is it because they have different historical backgrounds and etymologies?

Answer

Surprisingly, I wasn't able to find much information on this outside Wikipedia, even though it's popular trivia in Japan, but here's data from a book about color names:

In China circa 1000 BCE, the only kanji evidenced is white. By 820 BCE, this grows to white, yellow, orange-red, and green-blue-black. In 770, 赤 is no longer used for orange. In 750, 青 is invented for green-blue, and by 720 緑 is invented for green and 黒 for pitch black.

So much for the use of kanji, which presumably had strong influence on Japan when they were imported in the first millennium AD. What about Japan's native words? According to Wikipedia, they are the following:

Aka（赤）which shares an origin with "akarusi" (bright), "ake-/aku" (open), "akaaka to" (brilliantly/flaming) etc.

Kuro（黒）which shares an origin with "kurasi" (dark), "kure-/kuru" (darken/end) etc.

Siro（白）which shares an origin with "sirusi" (mark), "siru" (knowledge), "sirasira to" (obvious speech). Apparently the original word is Old Japanese "sirusi".

Awo（青）which shares an origin with "awi", the plant indigo, and which is claimed to be the antonym of siro.

Therefore, it is claimed, ancient Japanese had four colors: light-warm, dark-cool, distinct, and indistinct. In the transition to medieval Japanese, light became red, dark became black, distinct became white, and indistinct became blue-green. These comprise the four i-adjective colors.

The origin of midori is unknown.

Later, as another commentator said, "iro" was added to words to mean "the appearance of tea" etc., and later still color words were borrowed outright from English. 黄色, yellow, comes from the appearance of sprouts.

For more information, Wikipedia has some book recommendations at the bottom of its color names page.