Sand and debris from explosions. I've seen some mecha series use flexible armor to hide what they can't draw (like inner joint details) so as a personal rule, I try and use them only after I've worked out how the internals look and work and if they make sence.
Im quickly becoming a fan of your work. Im working more with the prisma pencils, and im finding that I am actually able to varry the colors in a subtle way as you do. Have you used the clear blending pencils by Prisma? I came across a couple the other day and wanted to know your opinion on them. Man your work is inspiring. Awsome, just awsome.
I can't say I've ever come across those particular pencil crayons. I find in some ways, I just luck out. My suggestion is finding a colour you like and testing it by shading with it on a peice of paper. If you get a smooth gradient and it feels like a stiff pastel then you have a pretty good one. Sometimes though you get a really plasticy type of prismacolour that picks up the subtle grains of the paper and doesn't shade very well. Really, I'd suggest thinking of your colour scheme ahead of time and testing their blending abilities before you commit. I learned this the hard way on some images you won't see in this gallery : )