Priming with simple black and grey/white; you can use Rust-Oleum Automotive Filler Primer to prime certain 3D prints. I’ve had hit-or-miss success with it… it can tend to clog easily if you’re not careful. You have to spray it in light layers and depending on your humidity it can sometimes stick to your model in a fuzzy mist.
I’m using a variety of red and purple inks, GW/Army Painter paints and washes to layer the effect of the gore, muscle and outer layers. Inks work well with primed PLA as it tends to absorb and spread into the medium. Since I’ve primed the models black to white the way the ink absorbs creates some shadow effects for me. It can sometimes create happy accidents depending on how the ink spreads YMMV depending on what you’re painting up; sometimes the random spread effect doesn’t work as well…
To create “fancy” gore: You can use a mixture of super glue, red acrylic paint and plastic desiccant beads from a used desiccant pack to make slimy egg sack textures…
Still not quite done with the chimney spore stacks but it’s coming along nicely.