This DOES NOT work with a box from the store. Those have the developer in them.
But if you buy the colour at a beauty supply store, you can add it to a cheap conditioner. I add a little as I need it, mixing it in the same bottle I use to color my hair. The ratio is 1 part color to 1 part conditioner.
Then I apply it, leave it on about 5 minutes and rinse with cold water. It will refresh color without damage, since there is no developer.
This works for all colors but best for black, reds, the "unnatural colors" and on bleached hair that has been colored darker again.
If your hair is more than one color, paint the color/conditioner onto the part that is colored and not to your whole head. I use a 1" paint brush, seriously (I like it better than the stiffer bristled hair color brushes.
It will not work with a color lighter than your own, of course.
For blondes, getting the same level as their color but with a violet base or a blue base will prevent brassiness/oranging.