In the following verse of this poem by Anna Akhmatova:
А здесь уж белая дома крестами метит
is белая a noun? If it is an adjective, I do not see the noun it describes. If it is a noun, what does it mean? I saw this question, but there the nouns are masculine and not feminine.
It's an adjective playing the role of a noun; it's called nominalisation. It's pretty common in Russian:
etc.
The adjective declension matches the noun it substitutes, so e.g. for «пациентка» it would be «больная».
In your example it's difficult to know exactly what «белая» stands for, but it must be feminine: «зима», «смерть» might fit.
In some cases, an adjective can also be a noun, the same in English. In this particular case, it's more like a cultural poetry slang that is not really used that often, where the word "white" [белая] is used as a noun meaning "death".
The other example on this specific case would be "bony" [костлявая], an adjective, that usually means "so skinny that the bones are sticking out", but in a poetic slang it also means "death", a noun.
Something like that :))
