What effect will henna have on brown hair?

Depends on the shade of your original hair colour really.
If you have light brown hair then you'll end up with a vibrant shade of reddish/orange. If your hair is darker then you could end up with reddish highlights.
Henna creates a reddish colour - what if you don't want too much of the red tones? In this case your best bet is to add indigo to your mix. Indigo, when mixed with henna, darkens your hair into a rich brown. Using more indigo creates a darker shade and more henna brings in the reddish tones. Any grey hairs will become red/orange unless you add Indigo.

Henna & Indigo can both be used as often as you like with no damage to your hair.

Henna & Indigo (sold at LoveHennaHair)are natural products with no added chemicals so an exact shade cannot be guaranteed.