10 REM wind the colour down or use a black and white television to see greyscale bars
20 REM The ZX Spectrum display is 32 columns wide, so we have 8 columns of 4 spaces
25 BORDER 0: CLS
30 FOR r=0 TO 21: REM There are 22 rows
40 FOR c=0 TO 7: REM We use the native colour sequence here
50 PRINT PAPER c;"    ";: REM four spaces, the semicolon prevents newline
60 NEXT c
70 REM at this point the cursor has wrapped, so we don't need a newline
80 NEXT r