THIS WORKS ON MY PREVIOUS COLOR THE CAT PROJECT. The way this works is actually simple, the backdrop is an image of the same sprite, but this time it's the opposite color, and then a sprite (which is half transparent) of the same image, but without alterations to the color, which combines the two images, causing the colors to be greyscale, you know how pen is always behind the sprites, but pen is also always infront the backdrop, so the pen covers up the background, but not the sprite, therefor the part you drawn at is no longer black and white. If you would like, I can go ahead and try making a demonstration.
Thanks to @MartinBraendli2 for his interest project https://scratch.mit.edu/projects/175039437/ I copied a few of his scripts, and I have learned his method, please take a look at his project, as I learned this from his project. @heldlaw made an interesting project with it aswell, if you liked this, you'll probably like this more I also realized that your pen should be the background color, for the pen to not be in the way. https://scratch.mit.edu/projects/115961813/
