You cannot technically attach units to other units in WarCraft 3. You can, for example, attach the image of a peasant to the "hand" of a Mountain Giant, but that peasant is not technically a unit, so any custom colours you added to the unit (in Object Editor) won't do you any good, because you're only attaching the game's default unit model for the peasant. You cannot resize the attachment, you cannot change its color, and you cannot reference it in triggers. It's essentially nothing.