Yes, RC Color is safe to use over other brands (but not henna). It is easy to do on your own, it mixes to a mousse foam. You won't get green hair! LMK which swatches match your natural color, dyed color, and what you'd like, and I can help you choose a color too.
Remember to always choose a couple of shades lighter than you think you want, as most haircolor goes darker on curly hair.