Hi Mahalialee4,
Either the neutral henna or natural henna can be and are being by lots of people with relaxed hair. The neutral or natural henna are used as a process separate from the relaxer. People that I know who are using the henna to both condition and color their relaxed hair have done the henna application and then the relaxer a few days later...and also some have done the relaxer first and then the henna application a few days later. It does not seem to have matter...the outcomes were positive either way.
The issue between henna and relaxers remains that you always want to be sure that you are not using any form of a henna compound (the culprit with the metallic salt included) and using natural henna or neutral henna. The best way to know if it is natural or not is truly to know who you are purchasing it from or know the brand very well.
And as always, do follow up the relaxer and henna applications with a good moisturizer and/or moisturizing oil for hair that ranges from normal to dry.
Take care.
HennaJoy