SETSCREENCOLOR color
SETSCREENCOLOUR color
SETSC color

Sets the screen background color to color and changes the entire screen to that color. The color input may either be a color index or a color vector. A color index is a number from 0 - 15. A color vector is a list of [red green blue] intensities, each in the range of 0 - 255. For an explanation of the color input, see the section on Using Color.

SETSCREENCOLOR [0 0 0]
SETPENCOLOR [255 255 255]
REPEAT 4 [FORWARD 100 RIGHT 90]
CLEAN