They were usually painted white but you could do them black if you want to, don, worry about the rivet counters as most of them are nothing anyway. Rule of thumb for those who wish to paint low vis stencils on camouflage, if you are stencilling onto green or tan always do the stencil in black and if you are stencilling onto black do the stencil in green, that is how the book says to do it.
Cheers
Rod