Rock Molds are made of a non-latex polymer.
Yes, the black wash can be lightened, as long as it has not been sealed already with Scenic Cement. Flood-spray the casting with hot salt water (1 tsp in 6 fluid oz of hot water). Wait one minute, and spray again if needed. Dab with paper towels to remove the liquid. The base colors will remain on the rock, but the coloring will be much lighter.
If you would like to start over completely, paint the casting with White Earth Colors Liquid Pigment. Dilute the White Pigment 1:1 with water.
When satisfied with the coloring on your casting, spray with Scenic Cement to seal the colors and let dry.