At this point, they looked much better and I could have easily stopped there, but I wanted to add some more interest to them:
I continued to age the skull with some more dark brown and even a little black. I then used some dark red and black craft paint to make some bloody areas around the eyes and a couple of wound marks. I used both a wet and dry paper towel to blot them out a bit making them look a little streaked and clotted. I used some of the blood in the teeth area too which added a great depth to them.
The last step is to use a very fine brush to make the 'fractures' in the skull with some black craft paint. Just make some jagged lines that get thinner as they spread out. I tried to make the skull on the right here look like it'd been smashed with a hammer. and the one on the left like it'd been stabbed with a kitchen knife or maybe chopped with a machete?
As you can see here, they don't need to be perfect up close, because no one is likely to be studying them this closely. Use your imagination here, there's so many possibilities! If you scroll back up to the very first picture you can see there where I used this technique to make the eye socket look a little broken.