Why does ''The color of a car.'' become 'car color' as a heading and not 'color car'?

In English

we normally put the modifier (e.g. adjective)

before the noun being modified.

In "car color"

we are talking about "the color of the car".

In other words: "car" is the modifier, and that's why it goes first.

Likewise in "flower structure"

were are talking about "the structure of the flower"

and so "flower", being the modifier, goes first.

These are examples of using a noun as an adjective.  In english, we place the adjectives in front of the noun.  bar tender (so used it became a compound word bartender), car driver,  garbage man, baseball fan, food aficionado, nerd scientist, on and on and on.

because it's English, not French or Indonesian

and for proper English, those words need a hyphen, as they are compound nouns:

flower-structure, building-integrity