Might I also suggest the "Saran Wrap Trick?"
-In this case, your basecoat is black.
-Hit it with a gloss coat.
-When dry, spray (or brush heavily) the glossy surface in question with the secondary (tertiary, etc) color you want.
-While still quite wet and fresh, take a piece of plastic wrap (or plastic grocery bag, etc) and either put it on the wet surface smooth (then scrunch it up, to create veins) or take it all crumpled up, and press it against the wet surface (again, to create veins).
-Hold 5 seconds, pull straight off the surface, and let it dry. Do it multiple times, with multiple colors.
Finish with a clear coat and you're golden.
LINK!
http://www.youtube.com/watch?v=tZQ434j9m_0
In this video, he uses an airbrush, but it can also be done with a rattle can or, if you prefer (or the surface is quite small) you can just load it up with a paint brush.
No word of lie, I just marbled the armor plates of my Cryx in black/red with splotches of black/gold.