OpLink Security has applied for a trademark for their App Icon. They decided to leave their color options open, and filed for a black and white combination instead, even though their iTunes Icon is in color. This way, they can change color schemes throughout the life of the trademark without jeopardizing scope of TM protection.
Indeed, if you are not sure what colors you want to adopt, or have a whole series of App Icons with variable colors, black and white is a safe option. If you have a definite color in mind, then you should by all means protect the mark with its color.
