It's okay but...
It seems like a great idea. But firstly you should offer more colors, and then to add maybe you should instead put a bar on the side with a bunch of colors on it. Or perhaps add in a color mixing function like that of ms paint so as to give a wider variety of colors. Thirdly maybe you should give a tutorial on how to make a sprite, or perhaps give a basic...outline for sprites. or give ideas for possible sprites. maybe you can also add in an animate feature as well as some calm music to listen to while people use it.
You have a great thing started, keep at it :)