Just wondering from any experienced painter out there, I thought it would be quite cool to paint my cold ones black but i don't know what highlight/wash to use and what color to paint the underbelly as it is usually lighter than the scales. Thanks in advance for any help!
Depends on how dark overall you want them to look. I think black and a bright colour might look a little bit odd depending on how it was done, so possibly some mid greys.
black is HARD to do. i suggest mixing in bleached bone, or even elf flesh for highlights. do NOT use the pre-blended greys for this, and do NOT use white as a mixing color. you get very interesting results if you experiments with colors such as vomit brown or goblin green for mixing color. try it out. you can use while as a mixing color, but it is insanely difficult to pull off and get nice results. the same goes for blue. if you are at a non-professional level of painting though, i recommend elf flesh. after you are done with the highlights, bring it all together with a wash of badab black.
You can always do the privateer press(where I learned it from) method of highlighting up from black. Basecoat in black, then a dark mix of black, enchanted blue, hawk turquoise, And for the final highlighting, mix in greater amounts of space wolves grey to blend it up. it ends up looking(if you do it right) something like this