Not sure if this will work, but: As the digidoc buttons are flexible - you'll need flexible paint (So it dosen't flake). When I used to build RC Cars, you had to paint the flexible body shells with polycarbonate paint - otherwise it would flake with the inevitable impacts. I know TAMIYA does this paint, and you should be able to pick up a small pot of black at a model shop.
The only stuff I can think of is available at hardware stores like Home Depot...it's a "rubber paint' used to increase grip on tool handles, etc.. More like a liquid vinyl--usually black, sometimes red in color. You would have to be careful against build-up when applying it to those buttons.
