I'm just about to paint a rockwall and was thinking about using normal matt emulsion paint (the type you'd paint your living room walls with). This type of paint is ok for pets and kids to be around when it's dry so it should be ok for geckos. I'm willing to take any advice about this though - you can't be too careful.
My rockwall is covered in grout too so whatever I end up using should be safe for you too.
Most of my larger hides are painted.....Latex textured type spray cans used.....Just don't apply in the area of the geckos and let it dry for a couple days.I have had no problems other than a little rub off from gecko rubs while shedding. Read the cautions on the can. Mine came from crafting areas.....things for kids and animal projects there. I usually carpet the tops but here is one not carpeted with a feeder cup in the lid. You might also consider a tunnel made from PVC pipe(hardware store) in photo...also painted that they love to use. Sleep in it and rub off shed. They will go in and do a 180 and go in circles to rub off the skin. ***Take care. HJ