What do you want to see in the API?
The data for player avatar pieces should be a small object that includes the id of the piece and the colour hexa value of the that piece.
The currently existing colour field in the Avatar data should still remain there as it is used for the skin colour (like it is currently)
This data should obviously be possible to be given to the server as the Avatardata is currently and should be returned when avatardata is returned.
Any additional info?
This colour data is used to colour the specific pieces with specific colour giving the player more customization options.