Play around with the sliders. Click the stage to hide/show the sliders.
I though @haruni459's original project was such a great demonstration that I just had to make a remix adding CMY and allowing the backdrop to be inverted. If you need an explanation, check the following projects: Color Illusions (Part 1): https://scratch.mit.edu/projects/550756447/ Color Illusions (Part 2): https://scratch.mit.edu/projects/551106266/ RGB Monitor Simulator: https://scratch.mit.edu/projects/552549282/