I don't know about henna, but you can shampoo or condition wash after RC's Color. I either shampoo with the one that comes with the color, or I rinse really well and condition. You don't need to scrub or anything, it just helps rinse out the excess color off the scalp.
SF Bay Area, CA * "The Angel-Goddess-Guru of Haircoloring"
3b/c/a mix. medium texture, low porosity
* pw: just4curlies * My Motto: Strand Test!
some hair pics -- gone, but never forgotten.