Personally, I wouldn't use it. Here are some links to threads about Rainbow henna:
The second thread is kind of long, but informative. It looks like the ingredients for some of the rainbow hennas are dubious. Real henna is red, and yet they offer a "rainbow" of colors. Also, they say that the product coats the hair shaft. Real henna doesn't do that - but compound henna (bad, chemically enhanced henna) does.
I would advise either ordering online or going to a local Indian grocery store, and asking one of the ladies if they use henna and which brand they use. They usually carry henna from India. Even then you have to be careful, which is why I suggest asking for the brand that they use on their own hair. And when they hand you a box, make sure it's not dusty and old. The powder itself should be green and smell like fresh-cut grass.
I order from www.mehandi.com
. Others here get henna from www.fromnaturewithlove.com