The model itself is a resin casting. Here's a pictures of my painted version of the basic model.
Now Lovecraft had a thing about tentacles so it would be poor form not to give one of these at least a few :)
Here's a second models with three tentacles from my Son of Yog-Sothoth kit.
I painted the models with Vallejo paints. Basic method was to drybrush the whole thing in progessively lighter shades of green. A little red mixed with green was painted around the mouths then the whole this was washed with thinned black acrylic. I kept applying wahes until the body was suitably dark. Once that was done the eyes were picked out with a progression of mutation, sick, escorpena and livery green. the the whole thing was given multiple coats of gloss varnish.
Finally a couple of pics of the shoggoths next to a figure for scale. "Tekeli-li Tekeli-li"