There isn't any good way to get acrylic paint to stay on the tracks.  I recommend a good enamel paint.  It has a bit more bite and will stick better.  It will still flake off if they are handled and flexed too much though.

A better option is to replace the rubber band tracks with indi-link plastic or metal ones.

Yep, what Gino said, on both counts.

HeavyArty

There isn't any good way to get acrylic paint to stay on the tracks.  I recommend a good enamel paint.  It has a bit more bite and will stick better.  It will still flake off if they are handled and flexed too much though.

A better option is to replace the rubber band tracks with indi-link plastic or metal ones.

 

Then prime them with enamel or lacquer first, and hope for the best.

It's not a perfect solution but you might try washing the tracks very well using dishwashing detergent and scubbing with an old toothbrush. When dry spraying them a few light coats of 'rubber bumper paint' from the auto parts store as a primer. Then paint them whatever colour you want and try not to flex them too much putting them on. 

I've had pretty good luck overall but some tracks will have paint flake no matter what. All you can do is carefully touch up the spots with a brush. 

Tojo72

Are these "old rubber band vinyl tracks" or from a more modern kit ? New Tamiya tracks are very paintable as are the Dragon DS Tracks. what are you building ?