10 CLS 
20 FOR i=0 TO 255 STEP 5
25 COL 0,i-5,1
30 PRINT "B";
40 NEXT i
50 FOR i=0 TO 255 STEP 5
60 COL i,0,1
70 PRINT "C";
80 NEXT i
90 FOR a=0 TO 255 STEP 5
100 b=255-a
110 COL b,a,1
115 PRINT "D";
130 NEXT a