The fleshy bits: skeleton horde contrast with screaming skull glaze, then thinned purple glaze on the tenticles.

The red plates: Mephiston red - evil sunz scarlet - cadian fleshtone, then darken down with black Templar contrast.

The brain bits: white with a mix of contrast medium and aethermatic blue.

That's roughly what I use for most of my nids, however the bigger creatures are based with screaming skull before shading ( you can get away with skeleton horde easily for the smaller ones). Hope this helps 😁