Doesn't matter whether it's an iron or ferrite core.... it's the cover that's rusting. If the cover slides off easily, you could clean them up and paint them any color you'd like.

Ferrite :
1 : any of several magnetic substances that consist essentially of ferric oxide combined with the oxides of one or more other metals (as manganese, nickel, or zinc), have high magnetic permeability and high electrical resistivity, and are used especially in electronic devices.