I admit that. I also realize that tguvot is actually arguing in my favor, as he said that coffee color is distinct from brown, and therefore the inference is that they aren't synonymous. I would summarize that they are conceptually different, as "brown" is a real color, whereas "coffee color" is a marketing color.

> "кофейные брюки" is totally ok. everybody will understand it.

If the context is clothes, people would likely be able to guess, sure. But consider another example "кофейная чашка" ("a coffee mug"). In this context, it would most certainly be interpreted as "a mug for coffee" and not as "a coffee-coloured mug." In other words, you must include the word "цвет" ("color") for it to be correct and unambiguous.

> it's just the way the russian language is. you can abuse it, you can come up with words that do not really exist in language and make no sense, yet, everybody will understand what you meant to say

I don't think this is unique to Russian. I'm sure you can do the same in English and Japanese at least.

It’s fine as an occasional stylistic choice, but using it repeatedly as a regular synonym for brown is a pragmatic and collocational error. The meaning is clear, but the wording is marked, and overuse makes the speech sound odd in everyday contexts.