If you work with frosting long enough, you are going to mis-mix some. A few of the Americolor brand colors are ones that get really dark, really fast. I'm looking at you, peach, leaf green, teal and fuchsia. Whenever I add color to my frosting I like to squeeze a bit in the bowl and stir, hoping it will render the perfect shade that I needed. I have done that more than once with leaf green. I want more of a kelly green and I end up with forest green.
Everyone's first inclination is to add more white frosting. Will that solve your problem? Yes, but then it will create another problem of mixing way too much frosting. If you don't want to end up with a gallon of green, you need to follow these steps.
1. Take most of the wrong color out of the bowl and put it into another bowl.
2. Add white to the almost empty bowl and stir. Now it is too light.
3. Add a spoonful at the time from the bowl that is too dark and mix. Repeat until you get the appropriate shade.
4. Use the leftover frosting that is too dark to make black.
There. You have fixed your problem with minimal waste. Need a better visual? Watch this link.