Or you can go the long route by changing the INK color and make a box the same size as the screen. I don't actually recommend the long route but you should know all the options.
` Change the color to dark green
ink rgb(0,100,0),0
` Make a screen sized box
box 0,0,screen width(),screen height()
` Wait for a keypress
wait key
The box command also allows color changes in each corner of the box to make gradient boxes.
` Make a screen sized box with a gradient
box 0,0,screen width(),screen height(),rgb(255,0,0),rgb(0,255,0),rgb(0,0,255),rgb(0,255,255)
` Wait for a keypress
wait key