Here is the color treatment I used on the Aliens project I recently completed.
I hope this is of some use, if not to be copied, at least to be adapted:
Prime black (P3 Black Spray Primer)
Overbrush turquoise (GW brand Kabalite Green and Nurgling Green)
Airbrush olive green + a drop of pale blue from "top down" (P3 brand Thrall Green and Underbelly Blue)
Light drybrush of middle metallic silver (GW brand Necron Compound)
Water effects + a tiny bit of white over the smooth alien head (Vallejo Still Water and GW Skull White)
Black wash over the claws (Secret Weapon Miniatures brand Black Body Wash)
Bright silver metallic on teeth (GW brand Mithril Silver)
Dip in Army Painter dark
Aside from the base treatments, this is the recipe I used for the creatures. I recognize that each of the Alien films used a different color palette, so I tried to base my treatment on the prototype paintings done by H.R. Giger:
I thought that would be the most "accurate" source material, seeing as how he was responsible for the design of the original Alien.
The models themselves are mid 90's era Genestealers by GW, with Chapterhouse "Xenomorph" heads, and an assortment of Secret Weapon Miniatures"Alien Infestation" bases. I used about 40 steel decking bases, 40 bases with the alien hive growing over the decking, and 40 pure alien hive bases.
Here is the KR Multicase army carrier that the whole collection fits in:
I have yet to see Prometheus, but my prehensile tongue is flickering with anticipation.