The color you see when you press start isn't one, but two. The colors are actually yellow and red. If you zoom in enough, you can actually see the two different colors. Computer screens work the same way, but with 3 colors, offering a far greater variety of colors. Due to Scratch's limitations, the resolution for this is much lower than a normal screen.
i made this project by using the pen feature. The code makes it so that the sprite puts down a color, moves over one and places down another color. It does this over and over, specifically 6000 times, until it reaches the final result of this. Update 11/12/23: I made the code better and now its a much higher resolution. (btw don't click full screen, it'll look much worse than it should, also the look of the color is no longer to chance.