It's so cute! And a very clever use of the color restrictions. Like darngeek mentioned - I also didn't even notice the restriction.
My biggest point of criticism are the controls / the input scheme. Jumping feels a bit uncontrollable. The movement in the air is too fast. And using 'Z' may be good on english keyboard layout. But it can be a problem for non-englisch-layout-players. E.g. I have a german keyboard and my Z is where your Y is.
But beside this, you made nice game with a unique graphic style. Thumbs up!