Henna!
Henna Gloss for blonde hair
Measure into a glass bowl
3 teaspoons of Jamila henna
1/2 cup of hot chamomile tea
let sit for just 15-30 minutes (for dye release,* you can wait longer for a darker/brighter color)
add 1/2 cup or more of conditioner
Apply to hair, let it sit for 30 minutes, rinse, wash and condition hair as normal.
*Dye release is different for different types and ages of henna. Other things can also affect dye release, so you'll have to pay attention to the henna. Some release in 5-10 minutes, others take an hour or more. The henna should begin to turn from green to brown. The longer you let it release, the stronger and darker the henna will be. For very red hair, let the henna release dye overnight. For blonde henna, a short release time is recommended.
The Hindu henna instructions