Nowadays I justify new armies by saying "these will paint up so fast" by using some simple trick or other. In this case it was to model the army after the "Alien" movie, and paint in basically monochrome with washes and gloss varnish to punch it up. Originally I was going to model the Alien heads with greenstuff, but then I came across Chapterhouse Studios' "Xeno heads" and the project just became waaay easier...
Chapterhouse Studios (http://chapterhousestudios.com/webshop/) sells a variety of different resin bits to suit 40K enthusiasts - mainly Eldar and Space Marine stuff, but there is some love for Tyranid players too, with resin boneswords and lashwhips, and most importantly XENO HEADS!
So I immediately fired up an order for 60 heads, figuring that would be enough to outfit the smaller models in my army. One of the difficulties of ordering, though, was the fact that nowhere on the website are there any pictures of how the heads fit on actual Tyranid models, to judge their scale. I imagine this may be because of intellectual property issues but it made me a bit anxious as to how they would look, since there are no pics around on the 'net either. Until now anyway ;-)
Turns out I needn't have worried. The heads arrived, after a fashion, and I eagerly tore into the package. The heads come on sprues of 6, with 3 examples of each variant per sprue. The cost is roundabout $1 per head. Well worth it in my opinion; the heads are wonderfully sculpted and the casting are crisp and remarkably free of flash. One of the variants has a subtle socket in the bottom that is sometimes helpful in fitting, but the resin is easily trimmed as needed.
The heads really suit the larger Tyranid models (Raveners and Warriors) but I think they look OK with Genestealers, Hormagaunts and Termagants also. Check out the photos and you'll see what I mean.