There are cases when, say, you want to depict a broken, thrown, or disassembled track.

The rollers were steel, and probably got assembled in the tank in their anti-rust grey primer. The outer faces might (?) get some camo paint on them as overspray, but were quickly coated with grease and dirt - similar to railroad freightcar wheels - while the running surfaces would be scraped clean and shiny metal. But let the tank stand overnight and all that shiny bare steel would develop a bloom of orange surface rust! So, how you paint them really depends on the situation you are modelling. If being assembled at the factory - grey primer all round. If it has been running and has just thrown a track - shiny bare steel. And if left any length of time - rust.