You can paint the aquarium rock after it has sufficiently hardened. Any type of non-toxic paint can be used. Oil-based paints can be toxic to fish and taint the color of aquarium water. Rough edges of any aquarium rock formation should be sanded prior to painting.
Buildable Rock Slime
Kids love rocks. And kids love slime. But what they will REALLY LOVE is Rock Slime! Buildable Rock Slime that actually holds it's shape. (DIY kinetic rock)