Invest in an array of gel or paste food colors. They are concentrated so you will get vivid colors without changing the consistency of your frosting. You can buy pastes and gels online or at craft stores that carry baking and decorating supplies.
Learn color-mixing basics. Red + yellow = orange. Blue + yellow = green. Red + blue = purple.
Play with colors! When you have a little extra frosting, play with the colors. You can save the frosting by covering tightly and freezing. Or you can always find a graham cracker to frost and enjoy.
Add color gradually. It’s easier to make colors more intense than to go the other direction.
Dark red frosting can be a challenge. Since the color darkens with time, color frosting the day before using.
Lemon juice and cream of tartar are acids and will chemically change colors. Avoid these if you are making frosting from scratch.
Water in frosting can cause color changes. Avoid using water and instead, use milk for liquid if needed.