"A gray hat."
Translation:Un sombrero gris.
Curious as to why a fish burger (una hamburgesa de pescado) contains the word "de" but a gray hat (un sombrero gris) does not . . .
Typically speaking, when English sticks two nouns together [x y] and uses the first as an adjective, Spanish will use a [y de x] construction. When there is an adjective, you don't need to add the de.
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.