I have been painting a lot of cracks and metal plates lately, and the thing they have in common are dark lines. I have tried several lazy ways of maintaining the dark lines while highlighting the panels between them, and most of them haven't worked very well. This is the system that I have been using lately.
I am showing the Magewrath Throne here, but the technique works well on metal plates as well.
1. After the basecoat I wash down all of the cracks. Washing over large flat areas is a waste of materials.
2. Next I outline all of the major cracks with my midtone color. In this case, it is dawnstone on the top skull and stormvermin fur on the bottom.
3. Using a drybrush and the midtone, blend the outlines from the last step towards the center of the panels. You still want to maintain some of the colors from the basecoat.
4. Now i outline the panels again, this time brighter, and with a finer line. On this model, I am using Administratum Grey.
5. The final step is to drybrush over the entire model. On a mechanical model, this step picks up rivets and corners. On an organic model like this, it picks up all of the smaller ridges and details of the rocks.
On this model I am using Longbeard Grey on the top half, and Terminatus Stone on the bottom.