The Color Replacement Brush n PSCS is very cool. Here's how I changed the black material to purple. 1. Dup the layer; 2. Select color from swatches; 3. Select Color Replace Brush (it is located under the Healing Brush); 4. Paint over black material; 5. Lower opacity on the painted layer....that's it.

No more wondering if you can get the amount of paint, the color and the right layer correct. Adobe really made it simple. Give it a try.