One word, Henna! They used to only be red but now come in all colors. They brighten your hair... but if you don't get a good color match or choose something ridiculous, that's what it will look like... available at a beauty supply. I use it over my color to get a more natural look because something out of a bottle looks artificial (might be OK if you're not a natural
). It gradually washes out but is actually good for your hair so you can use it as often as you want. I use Hennaluscent.
Watch those grays. They attack when you least expect them
For those of you who aren't 'naturals', it comes in all colors and brightens all hair. Also gets those hard to cover grays when used right after coloring. My sister-in-law uses burgundy over her black hair for just a hint or glow of highlight. It shimmers in the sun.