What part of speech is 'white'?

The noun 'white' is a concrete noun, a Word for a color, a Word for something that can be seem.

The noun form for the adjective white is whiteness, also a concrete noun.

Because the noun or the adjective 'white' is a Word for something visual, something physical, the noun forms will always be concrete.

A concrete noun can be used in an abstract context such as a white lie (meaning a trivial lie, not a lie about something that matters).