Use the mouse to draw in the canvas. Use the color selection panel to choose a color. Use the arrows (On the screen) to change the size. Use the X to clear, and use the paint bucket to fill the canvas with a color. Feel free to remix with your own colors! Find the Color Selection sprite, find the long chain of purple blocks, and change the last 3 numbers in each one.
I saw something like this in hopscotch and thought, "why don't I make this in Scratch?" Purple Hawk made it in Hopscotch.