You can use Jolene bleach. Mix according to the package instructions, apply and really work it into your brows, then watch it like a hawk. Keep checking the color every minute, and the second it looks right, wipe it off. Be extremely vigilant, otherwise you'll end up with blonde or white brows. It's best to be very conservative with the color; you can always reapply bleach later. You don't have to worry about "frying" brow hair because it's so short.
This will work well if you're looking to lighten darker brown to a light golden brown. If you want a light ash brown, you'll probably need to discuss it with your colorist.