As long as you keep your henna warm-colored rather than going toward burgundy, I don't think it should clash too much.
I don't have olive skin, though I have brown eyes and generally a warm tone. My hair is maybe about medium brown, and the henna is apparent depending on the light (under fluorescent light, you don't really see it, but in incandescent or natural light you can). I have pictures in my album-- the older ones have my natural color (though I think it's slightly darker now as those pictures are old), and then there are a couple from a couple years ago of what it looks like in sunlight with henna.
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