To change the colour select a colour in the "Colour" section. To change the thickness select a thickness in the "Thickness" section. To erase anything click "Eraser". To turn off "Eraser" click it again. To erase all the colours click "Erase All". To colour the next picture click "Next Picture".
This is the upgraded version of the project "Paint". Link: https://scratch.mit.edu/projects/992852530