I've used crayon on a 10/22 before and it worked good. Not sure why I'd do it to my HK's though.

Crayon, specifically Crayola, works the best.

and you want to do this because??

Cause it's cool of course!

I don't have an H&K, but I do have a Ruger, and one of the more eccentric things I've done is to fill in the recesses of the logo on the bottom of all my mags with Bright Sights. It actually works pretty well; it makes the mags more visible and I can use the color types to identify the ammo in them (the mags holding my Federal defense rounds are painted with the Ghost Glow luminescent colors). They don't make white (the colors are designed to cover up white sights) but the Ghost Glow yellow would give you a very contrasting color that would glow in the dark.