Why is the sentence 「車はあかです。」 not 「車はあかいです。」? Since 「あかい」 is an i-adjective.
The primary colors in Japanese are all i-adjectives, which agrees with the idea that i-adjectives tend to represent more basic concepts than na-adjectives.
あかい akai red あおい aoi blue きいろい kiiroi yellow しろい shiroi white くろい kuroi black
あかいくるま a red car くろいペン a black pen
When you want to say that something is some color, you must use the no-adjective (a noun) equivalent of the word.
あか aka red あお ao blue きいろ kiiro yellow しろ shiro white くろ kuro black
ボールはしろです。 The ball is white.
Other color words are normal no-adjectives, so these five are really the exception rather than the rule. Fortunately, the rule itself is without exception: when using primary colors to modify a noun, use the i-adjective form; otherwise, use the no-adjective form.
You would also use aka to refer to "the color red", which is similar to how color words in English are used as both nouns and adjectives.
And finally, the noun forms are also used in some compound words like :
あかワイン red wine くろねこ a black cat
I hope it could help.