List of paintable items
From TF2 Wiki
Most hat and hair can now be painted. Halloween Masks can't be painted or the Modest Pile of Hat, Towering Pillar of Hats or Noble Amassment of Hats. The Superfan from the Monday Night Combat promo items can't YET be painted but, in the future might be able to. The Polycount hats from the Mann-Conomy update can't be painted. Hatless hats can't be painted aswell.