Full Idea
Whiteness is said to exist not because it subsists in itself, but because by it something has existence-as-white.
Gist of Idea
Whiteness does not exist, but by it something can exist-as-white
Source
Thomas Aquinas (Quodlibeta [1267], IX.2.2), quoted by Robert Pasnau - Metaphysical Themes 1274-1671 10.2
Book Reference
Pasnau,Robert: 'Metaphysical Themes 1274-1671' [OUP 2011], p.184
A Reaction
It seems unavoidable to refer to the whiteness as 'it'. It might be called the 'adverbial' theory of properties, as ways of doing something.