I've come across this beautiful description in the song a character sings in the book 1 of TLOR, 'the forge's fire is ashen cold'. I immediately grasped its meaning, but I got curious about that kind of collocation made of two adjectives like that. It doesn't feel like those descriptions where the two or more adjectives are separated by a comma, such as 'it was a musty, lingering smell' attributively or 'the mountain peak was icy, unyielding' predicatively. It feels more like ashen is modifying cold but it's not an adverb, and it works, despite my being unaware of the grammar going on there. It sounds more beautiful to me than 'ash cold' just like 'fiery hot' might to someone than 'fire hot'. I'd like to know more about that kind of collocation and how it works.