So you, or they were already, glued the treads to a tank before painting them.  You don't want paint meant for the treads landing on the hull of the tank.  Here's a tip.

JJ typing,

Time to paint the treads.  They were already glued in place before I started airbrushing the hull.

Don't want the paint from the treads messing up the finished hull.

Enter blue tape, tamiya or painters tape.  Used my finger nail to applying the tape.  Apply the tape right next to the tread.

Army Painter Mat Black for basecoat.

Drybrush Leadbelcher.  Then remove the tape and you're done!

Client wants the pads of the treads painted black.  Guess actual tanks have a rubber pad on the tread.

slainte mhath