Organic/natural paints are water based and free of VOC's and other toxic vapors. They would be your best bet. I would be careful with foam though. A gecko may go after a cricket and bite off a chunk of the foam.
Non-toxic paint such as tempra or acrylic should be fine, it's what I use for my hides. In order to not have any foam chunks bitten off make sure you grout the foam then paint, then use a grout sealant that is water based. Then simply let it dry and air out!