Does anyone know if Henna can be used over previously
coloured/chemically treated hair?  I used to highlight
my hair blonde and now my darker roots are coming
out.  I thought about just letting it go.  I don't want
to put anymore permanent dye on my hair, as I just
have gotten my hair back in good condition again
after the last highlight job.(In May).  I thought maybe
Henna could be used to both colour my hair and  impart
some body to my superfine hair as well, but I don't know
who to ask.  I don' t want to wind up with some strange
colour.   Should I ask a beauty supply person?

Babyfine, if you decide to go through with this, whether you're doing it yourself, with help at home, or at a salon, you should have a strand test on a small area that doesn't show before doing the whole head of hair.

Here's a website with lots of good info on henna
Henna For Hair

Henna used over chemically treated hair can turn it funny colors like purple, blue, or green. I suggest you strand test if you still want to use it!

apparently "true henna" is supposed to be fine (i am thinking about it too ... not that i am not satisfied with chemical ones, but the longterm use and toxicity of those dyes is scaring me more every day!)
it's the chemical additives and even dyes they add to many things labeled as henna that can cause the problem. avoid cheap henna i'd say.