Ok, since you used an acrylic base, you can use Pledge with Future Shine (used to be Future Floor Wax, by SCJohnson), but you can read everything you need to know about Future, here
The Complete Future
The Future is a self leveling acrylic gloss coat, so you can definitely brush paint it on. Even better, put the sculpture in a disposable tin pan, and pour the Future on top. Take the sculpture out of the pan, and let it dry on a sheet of newspaper. You can then pour the future from the pan, back into the bottle. Let the Future cure for 3 or 4 days.
Next, take some dark brown paint (acrylic is fine, as long as the Future is completely cured), thin it with water until it has a consistency of skim milk (a tad more viscous than water). Then, brush it over the entire piece. Wait a few minutes and then dab everything with a paper towel. The paper towel will remove the wash from the high points, but leave it in the recesses.
You may want to do this with a few different color paints, as long as they are darker than the base coat. For something like a facehugger, I'd go with with brown, hull red and definitely no lighter than dunkelgelb (it's a WWII german armor color, sometimes listed as armor sand, but dunkelgelb, is usally darker). The lighter the color, the more random the application should be.
After that has dried (about 2-3 days between colors), give the whole thing another coat of future, then mist with a satin coat (satin clear can be purchased at any decent hardware store, in the paint section).
Remember, you're not really adding color, you're creating shadows. There's an organinc difference in an animal that has spotting or stripes for camoflauge, and an animal that just has different tonal variations in it's skin. Look on google images for facehugger, you'll see some really good examples.
-Fred