Yeah I went through a massive henna craze 5 years ago. I was henna'ing it every 4 weeks. It went beautifully soft and silky and shiny! But it is just a highlightener. I have darkish brown hair, and after a few aplications, you could see that it had a slight hint of red to it indoors. And in the sun.. wow!!! red sparkles everywhere, it was great!
It's actually not recommended for grey or blonde hair as it might go bright orange

That stuff stains skin so badly too, I had to put pawpaw oitment all over my ears, and my hairline, and I STILL needed to scrub my poor skin to get the orange out.
And just a word of warning, if you dye your hair at the salon while the henna is still in (or even at home with regular colours), you might get some nasty reactions with weirdo colour results, so be careful.
I have blonde highlights, and as soon as they grow out, i'm going to start with the henna again

I used to leave it on all day, keep it warm (intensifies the colour) and I used to add things like tea, paprikia, lemon and even cheap wine into the mix to get different shades

The wine made it go more dark red, rather than orange red, tea was a more golden/brown red, paprika was for BRIGHT red and lemon supposedly was to make it stick to your hair more.
And yes, it's only hair

No biggie, it will grown out. Let us know how you go!