click on the rainbow gradient to select a color, then click and drag on the white space to draw :) change the variable sliders to change the size or transparency of the brush, or to change the brightness of the gradient color selector swatch - press "e" on your keyboard to switch to eraser - press "p" to switch back to pen - press "r" to reprint the rainbow gradient if it gets covered - space bar or green flag to erase everything
updated the gradient color selector: https://scratch.mit.edu/projects/331424959/ to add size, brightness, and transparency :)