A statement which clears the graphics viewport and sets it to the currently selected graphics background colour using the current background plotting action (set by GCOL). The position of the graphics cursor is unchanged.
GCOL 128 : Set background colour to 0 (white). CLG : Clears viewport to white.
GCOL 4,0 : Sets plot mode to invert, foreground colour to white. CLG : Inverts the viewport.