Press the colour you want, then draw with it. To do rainbow, you need to press the rainbow button on top, then draw with it. Then to go back to normal, press the rainbow off button. Slide the bar to change the thickness of the pen.
Check out @LibDash, he just joined and it would make his day if you followed him. UPDATE LOG ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ V1.0: Released v1.1: added a 'clear' button