Cake will be purple if you add something to the batter that will color it purple. Vegetable coloring...mix a few drops of red and blue into the batter. Make sure you use a white cake mix, not yellow...or else it will be more of a maroon or brownish purple...not as pretty. Or, you could mix a little concentrated blueberry juice (it's really a purple-blue) into the batter...again, I suggest white cake mix instead of yellow. Blueberry juice in the yellow cake mix will be greenish! (Ever see those blueberry muffins...the blueberries have greenish cake around them.) You could also try mixing anything else into white cake batter that will color it purple...of course, make sure that whatever you use is edible unless your goal is only to create art! Good luck and have fun!