Well, the colours have been picked by mortals, so they cannot be considered the ultimate colours of truth. The logic is that Magic is like the colours of the rainbow in its many forms. So, we have the 7 colours + Black and White. Pink and Brown just happened. Pink definitely represents Azira herself. Brown also seems fitting for a dwarf. Black is related to absorption and white is a good colour for healing. Azure is because of Imagination. I assigned Mardolf this domain, because I wanted it to be more Assembly-like, while creation andd Imagination had been given to Geidrich. Green = nature. Red is an aggressive sort of colour. For some reason the connection between purple and knoledge magic makes a lot sense in my head, but I'm not sure why. Yellow and Blue were what was left! :P
So, it's mostly because their abilities, but as I said this categorisation is not universal.