Instructions
Play multi-player mode until you have reached level 13 prestige --- you will not be able to change your clan tag color until you reach this level.
Go to the main multi-player menu and select "Create a Class," then click "Clan Tag."
Scroll through the list of possible clan tag colors until you find the color you want to use. Press "X" while hovering over the clan tag color to see a preview of your clan tag. Press "B" to return to the multi-player menu. Your clan tag will be changed to the color you selected.