grammar - Using i-adjective color words

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.

Answer

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

Some examples:

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

Example:

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.