It's an adjective playing the role of a noun; it's called nominalisation. It's pretty common in Russian:
- рыжая == лиса
- молодые == молодожёны
- больной == пациент
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.