Click the pink easel to erase everything. Hover over the eraser to switch the pen to eraser mode. Put your mouse over a color to select it. Press to color. This is not mobile friendly. Here is the version that is: https://scratch.mit.edu/projects/1153783944/
@Gladizia made a project and told me to try and replicate it without looking at the code so I did. Credit to @Rakkoon_Guy for the computer icon