MODE 8:REM 640 x 512 pixel display mode: BBC BASIC gives 2 graphics points per pixel
REM (0,0) is the bottom left of the display
GCOL 1  :REM Select colour one for drawing
FOR row%=1 TO 4
  n%=2^(row%+2)
  w%=1280/n%
  py%=256*(4-row%)
  FOR b%=0 TO n%-1
    g%=255*b%/(n%-1)
    IF n%=16 OR n%=64 THEN g%=255-g%
    COLOUR 1,g%,g%,g%  : REM Reprogram colour 1 to the grey we want
    RECTANGLE FILL w%*b%,py%,w%,256
  NEXT b%
NEXT row%