Your logic here is faulty, and your assumption is incorrect.
Except for a handful of the most common colors (red, blue, yellow, black, white), most colors in Japanese are regular nouns, not na-adjectives. To use them in an adjectival form, where they are placed prior to the objects they modify, you must connect them using no, not na.
There are only 6 exceptions in colors in Japanese. 茶色 Brawn is also an i-adjective besides the 5 colors mentioned above. See the following thread. Conjugating colours | Japan Forum