I'll just use my color scheme, to make it easy:
1. First, I undercoat the whole model in red gore.
2. Then, using a fine detail brush, I put blood red on the edges of the armor. If it's a panel, just paint it on with the tip of th brush, very carefully. If it's a ridge, dip the whole brush into the paint, wipe almost all of it off, and run the side of it down the ridge.
3. Do details.
It works for me, try it and see if it works for you.