Full Idea
Four arguments for Platonism: 1) there are truths about redness (it's a colour) even if nothing red exists, 2) redness does not depend on particulars, 3) most universals are at some time not exemplified, 4) universals satisfy the criteria of existence.
Gist of Idea
Redness is independent of red things, can do without them, has its own properties, and has identity
Source
J.P. Moreland (Universals [2001], Ch.6)
Book Reference
Moreland,J.P.: 'Universals' [Acumen 2001], p.131
A Reaction
This adds up to quite a good case, particularly the point that things can be said about redness which are independent of any particular, but the relationships between concepts and the brain seems at the heart of the problem.