Hello R. Pope,
Concrete stains are easy to use and clean-up is a breeze. You can find concrete stains at Home Depot or your local paint supply store. They can mix a number of colors that can be seen in thier swatch brochures. A little stain goes a long way.
If your ornament is moldy or just plain dirty, consider giving it a good wash first. since it’s an ornament, I don’t see any need to give it an acid bath or anything. the natural staining will blend well with your creative expressions.