Before painting anything I like to do a search and see what other people have done. Not so much to copy their work but more to get ideas and try to put my own spin on it. There were scant few Formless Spawns out there and I really didn't like any of them so I just went with what I felt like doing.
Here is what I settled on:
I don't know if it comes through in the pictures but I painted Blood for the Blood God in the mouth and put several coats of 1:1 Vallejo Game Color Red Ink and Sepia Ink in some of the recesses so it looks like sores. Everything got a matte varnish while the inside of the mouths and the sores got a gloss in order to try and make it look wet and gross.