This very much depends on how much handling you are going to do on the tracks.
In an ideal world you should prime before painting but with metal tracks it very much depends on the end result.
Today, tracks have had an oil bath and so have a very dark, almost black look but during ww II they were very much just left the steel colour they were made out of - especially German tracks.
Friul are my choice of tracks and I usually put them together first and make sure that they fit and only needs a piece of wire to complete the run when put onto the vehicle.
What I am going to say next may bring a scream from may on here, but I never prime them. I use the white metal colour as my base colour and then apply direct one of these with a very light overspray to just darken them
Steel
Magnesium
Burnt Iron
Gunmetal grey
Occasionally I may use two of them as a contrast.
My main colour to use on friul tracks is AK's Xtreme Metal Steel to which they get an all over coat and then once cured and left for 24 hours A very quick pass with gun metal grey from the same range.
Both Xtreme Metal and Vallejo's Metal colour are my go to metallics. For me, though Xtreme Metal just has an edge!