Double-click the green flag. Press 1 to change to red, 2 to be orange, 3 to be yellow, 4 to be green, 5 to be light blue, 6 to be dark blue, 7 to be purple, and 8 to be pink. C to clear your work. To change the size, use the up and down arrow keys.
Credits to @yeuford for his awesome project that got featured on how to make a rounded square: https://scratch.mit.edu/projects/113441533