"A gray hat."
Translation:Un sombrero gris.
Because it's not a hat of gray. A fish burger is technically a burger (made) of fish. In English, you can make a noun into an adjective when needed (fish becomes an adjective), but you can't do that in Spanish. Since gray is already an adjective, it doesn't need to be turned into one, and so it can use the same form in Spanish.
Yes however the Duolingo dictionary doesn't seem to think so completely.
If you look up "hat" in the Duolingo dictionary, it lists only "sombrero".
If you look up "cap", it lists "gorro". Look up "gorro" and it lists "cap".
But if you look up "gorra", it lists "cap, hat".
So their dictionary shows "hat" to be "sombrero" AND "cap" to be "gorro" WITH "gorra" as both "hat or cap" AND apparently their algorithms sticking to a strict "sombrero" for "hat".
Perhaps they will someday change the algorithms after reading this. I have noticed they have commented as such in other discussions. Or at least add an explanation as to why not.
http://www.softschools.com/spanish/word_order_for_adjectives_in_spanish/ I quote "To summarize, typical descriptive adjectives almost always go before the nouns they modify, unless they are intended to convey a more subjective or poetic quality. As a beginner, you will usually place descriptive adjectives after the nouns they modify, but don't be alarmed if you see them placed before. Notice these cases so that you can begin to build an understanding of when it is appropriate to place adjectives before their nouns!"