Hey there, Chris. Welcome to Braille! Cant wait to see pics.
I use two-sided tape, attached to a plastic card, and stick small parts onto the surface. If a part is too fragile, I cut off the entire area of the sprue carrying the piece.
If you mean painting the road wheels, well you could always "muddy" up that part. Alternatively, you can paint the surface of the rubber black and just drybrush the rest of the road wheel (saves you from having to strain your eyes following the demarcation line between the rubber and the rim). Otherwise, theres just no way around it, my friend.
Hope this helps.