Like nathan and daisy said, it would be unfair to those who have already won the winter waltz tournament. The point of the tournament is that it's challenging, which gives those who put in work a chance to win something no one else has.
In addition, I'm not quite sure what you mean by white and black wand effects being the standard colors in the harry potter films. From what I can tell, each spell emits a different color (ex: expelliarmus being red), similar to how potterworld's spells are set up.