It states that it has the colour of your choice (for calculating income). Do you make that choice when the card is played and it becomes permanently yellow for example, or can you change it each round?
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.