For black spokes I use Model Master's black chrome trim. It is a nice semi-gloss black.
Depending on what era of TTD's you are trying to replicate, there are different greys you can use. If trying to replicate the originals from the early 60's, I use Model Master flat gull grey. It almost looks white or like a "concrete" color. If you are trying to replicate the latest versions of the TTD's, I would try standard testors flat grey enamel. This has a little tint of blue in it which should match the new wheels very well.
Also, for the aluminum edge, I use Testor's dull coat on the chrome to look like polished aluminum.