EcoColors Haircolor has successfully been used over SOME BRANDS of henna. If you are staying in red brown tones, it may be OK.DO A STRAND TEST and an allergy test according to the kit directions before applying any new color to your entire head. What color were you thinking of trying? What color and brand of henna were you using? What is your percentage of grays? Be sure to READ THE INSTRUCTIONS on how to color previously colored ends with a DILUTED formula. Some hennas may cause an inconsistent coverage on the hair shaft.