Does the School of Magic count as all colours, or do you choose one colour when it's played and it is that colour permanently?
I believe you choose each round.

In my version of Citadels (Fantasy Flight, 2009) there isn't a card called 'Mage Tower'.  There is however a very similar card called 'School of Magic' which text states.


  For purposes of Income, the School is considered to be of the colour
  of your choice.  If for you are the King this round, for example, the
  school is considered to be a nobel (yellow) district.


To me that sounds like the card you are describing with extra text clarifying which sound like it may be missing from your edition?  Also if you had to decide when it was build there should be some way of marking that and the game does not come with tokens for doing so.