I painted the first half of my walls (made another two in the evening last night, so I now have 6 total).
Here they are:
Before painting I undercoated the walls with a ever so slightly thinned 50/50 PVA and black paint mixture (the same black hobby paint) to primer and toughen the surface of the foam.
Hopefully, tomorrow I can crank out the next three.
Until next time!