One of the paralegals at work graduated recently, with a Bachelor of Laws. When I graduated with the same degree I wore a purple stole, and I just assumed he would too. Unfortunately he graduated from a different University which uses different colours!
It took 3 different colours of food dye to achieve purple - purple (due to the colour of the butter the purple food dye made the batter look grey), red and blue. Of course, the outside of the cupcake still ended up brown, as you can see, but the inside of the cake was a great colour.
I am really keen to try to make other coloured cupcakes, if I can think of a good enough excuse. Maybe green and yellow for Australia day.. or red and blue for a Melbourne Demons game.
I used the gel food colourings, so I didn't have to add much to the batter to achieve the colour - this also means I didn't have to compensate and add extra flour, though that would be something to bear in mind if you use liquid food dye.