Here's a quick and dirty tutorial on painting up a casting. I'll be using one of the copies of the small Cthulhu icon I first talked about back in November.
I like using a a three tone approach to painted finishes. That means the final intended color, in this case green, is actually a blend of three different shades. First comes the basecoat. It establishes the color foundation of the piece. Next comes the midtone, a slightly lighter version of the base that helps build dimensionality. Finally you apply the highlights using the lightest shade of all.
Here's what we'll be working with today. The three shades of green on the right are my base, midtone, and highlight, respectively. The two paints on the left, brown and a dull green, will be combined to give me a weathering wash that blends the other three together.
Here's a closeup of the basic black casting. Since it's matte black on a white background I've tweaked the contrast a bit to bring out the detail. That had the unintended consequence of highlighting specks of dust as well. The piece isn't nearly as dusty as it appears. Heh.
First we lay down our base coat with a very heavy dry-brushing of the darkest green. Since it's not much brighter than the black of the casting we don't get much "pop" from this layer.
Now we apply the midtone. It's dry-brushed on with a slightly lighter stroke than the base coat. You can see how it really starts to resolve some of the details thanks to the enhanced contrast.
Finally, we apply the highlight coat. This is what really brings out the details of the casting. You want to use the lightest of strokes to dry-brush this layer on. You want just enough paint to bring out the fine, high ridges of the sculpt. At this point things can look a little cartoony because you've intentionally built up a finish with a high level of contrast. Now we'll knock that back a bit...
...with the dulling wash of dark green tinted with brown I mentioned earlier. This blends the other three colors together and lightens up the flat black of the base casting to bring out the fine recessed details the brush couldn't reach. Once the wash is dry hit your artifact with a coat of matte fixative. One of the drawbacks of dry-brushing is that the paint can easily flake off with handling. Friction is responsible for depositing the pigment, and it's equally good at removing it. A protective coating will keep the paintjob in place.