Select a color in the bottom left corner.
Right Click to place a pixel.
Use the mouse wheel or + and - buttons to zoom in.
Ctrl + Right Click to select a color from the canvas.
Tap to place a pixel.
Pinch to zoom or use the + and - buttons.
Long press to select a color from the canvas.
Source code on GitHub.
ps. join the Hack Club AMA on Apr 16 to watch this being painted by a machine