Always use good quality colors: paste, liquid paste, or powder. Do not refrigerate colors, or add water to them. Store them tightly covered. You may thin the colors with glycerin if they seem too thick. Always stir the paste food colors before using them. Use a clean, dry toothpick for taking small amounts of color from the jars. When mixing blues, lavenders, and pinks, try to color only the amount that will be used immediately to help prevent fading. Milk used in the icing helps hold the color, also.
Add protein (meringue powder) to buttercream to help prevent fading of color - especially pinks and blues.
Powder colors do not bleed as badly as paste colors.
To get a deep color, mix your color and let stand for 24 hours. It will darken a lot when in a shortening-based icing.