I am new to keeping firebelly toads. I have kept various species of reptiles and amphibians before just not these little guys. I have two in a 29 gallon aquarium. I have live plants in there, gravel/moss/driftwood as a substrate. there is a small pond with moving water. When I got them from the store they were a nice green color on top, not the brightest but green nonetheless. Since coming home they have been black. Occasionally small green spots will show up and sometimes the whole back will become a dark green. The gravel in the cage is black. Would this induce a color change? I don't have aheat lamp in there, could this be the problem?
Any ideas?

Thnkas in advance
Rich