Howdy! I've been looking into the same thing. If anyone has more experience please chime in, but here are the things I've discovered for reptile safe paints and varnish/sealant.
For paints always stick with acrylic. Avoid oil or water colors.
For varnish/sealants always make sure it says non-toxic or low VOC.
Avoid oil based. Go for water based. But keep in mind water based does not mean water proof! Water resistant yes. Some may include a combo, usually found in concrete sealants.
Make it stone spray I've seen a few people use, should be fine as long as you seal it. I've seen make it stone specifically paired with Krylon Matte Finish waterproofing spray.
Other sealants I've had recommended to me or have seen used are;
*Minwax Polycrylic water based sealer (what I have and plan to use)
*Water based polyurethane
*Shield's All
SealKrete Clear Seal
Ghost shield lithi-tek 9500
Note *= sealants I've seen widely used on multiple occasions. The other two I have not found any examples of but have had it suggested. So you'd probably wanna look more into it.
Hope this helps!