You can find this product at salons and stores that sell professional products. If you have trouble finding it, you can ask your stylist to order it.
It's totally non-damaging and adds pigment, and takes 15 minutes. Just don't forget to wear gloves and watch out that you wipe up any errant mousse right away as it is so full of pigment it really stains fast.
Goldwell Soft Color
Goldwell Soft Color Foam Colorant reviews - Makeupalley
6RB might be perfect, or 4G. You can mix these shades easily as well.
I keep suggesting this because it really is so very good, and so many people have no idea that a product like this even exists.