Step 8: Skin and bones
Paint your alien's skin. I used a gray latex paint that I had bought off the mistint table. Cheap!
If your intestines appear too bright and mono-chromatic, apply a "wash". Dilute some black paint with water until it's quite thin. Then using a brush spread it randomly across the intestines - it will pool in the low places and leave the tops bright. You may want to do this a couple times letting it dry between. Instead of black, you may mix black and red, or brown and red, to get different shades, and use this technique multiple times to build up more "depth".
Coat the intestines with a gloss coat. You can use spray, or something like "ModPodge" as long as it's gloss. This will keep them all wet looking.