Uh, it appears I accidentally left in normal arrow key movements without meaning to I MEAN OF COURSE that it was completely intentional and I put it in as a way for players to be able to somewhat control their movement.
Ignoring that minor completely intended mishap the way it works is that there are three speeds, slow, medium, fast, so when you hit a red block it goes slow, yellow goes fast and green is medium, but medium gradually slows down the higher your score gets making it tougher as you go on. Challenge mode starts medium speed slower than in regular mode, so that's probably why.
P.S. WE WILL TAKE OVER THE WORLD