This is how I painted my tyranids (with a link to a quick tutorial how)-
Painting tyranids isn't really that complicated. First decide if you want low/high contrast between the carapace and the rest of the body, then pick some colors. If
GW doesn't make an ink in the body color you want, you can use washes over a white or black base coat- either way, thin, quickly applied pigments are your friends. It's really hard to go wrong as the models really lend themselves to easy drybrushing, layering, or dipping.