This was discussed once before so when I saw it I copied the recommendation and kept it for future emergencies. This is what I copied, not sure from whom:
"Buy a can of Dupli-Color Dark Shadow Gray (CX) BFM0360. It is a 98% perfect match to the wheels. Smooth the rash a bit with 320 grit wet sand paper - just the white part - don't scuff the good paint around it. Just try to knock of the sharp points to create a flatter surface for the paint. Clean it well, then spray paint onto a Q-tip and dab the paint onto the rash. Let it dry and repeat for about three applications and the rash will be almost invisible."
Edit: credit for this goes to @sptcoupe
"Buy a can of Dupli-Color Dark Shadow Gray (CX) BFM0360. It is a 98% perfect match to the wheels. Smooth the rash a bit with 320 grit wet sand paper - just the white part - don't scuff the good paint around it. Just try to knock of the sharp points to create a flatter surface for the paint. Clean it well, then spray paint onto a Q-tip and dab the paint onto the rash. Let it dry and repeat for about three applications and the rash will be almost invisible."
Edit: credit for this goes to @sptcoupe