Mental challenge: the rainbow

["Marc S. Ressl" <mressl@gmail.com>]

Hello everyone,

This code was posted by emphasis at comp.sys.apple2.programmer:

 10 A = 9200
 20 B = 13136 + 32
 30 C = 100
 40  TEXT : HOME
 50 DATA 173,87,192,173,83,192,173,84,192,173,80,192,208,251
 55 DATA 173,86,192,160,22,136,208,253,234,173,87,192,76,9,3
 60  FOR I = 768 TO 796
 70  READ N: POKE I,N
 80  NEXT
 100  HGR : POKE  - 16302,0
 110  HCOLOR= 4
 120  HPLOT 0,0: CALL 62454
 130  HCOLOR= 2
 140  HPLOT 0,63 TO 279,63
 150  HPLOT 0,62 TO 279,62
 160  HCOLOR= 6
 170  HPLOT 0,61 TO 279,61
 180  HPLOT 0,60 TO 279,60
 190  HCOLOR= 1
 200  HPLOT 0,59 TO 279,59
 210  HPLOT 0,58 TO 279,58
 220  HCOLOR= 5
 230  HPLOT 0,55 TO 279,55
 240  HPLOT 0,54 TO 279,54
 250  HCOLOR= 0
 260  HPLOT 0,123 TO C,123
 261  HPLOT 0,122 TO C,122
 262  HPLOT 0,119 TO C,119
 263  HPLOT 0,118 TO C,118
 270  COLOR= 0
 280  FOR I = 0 TO 39
 290  VLIN 0,39 AT I
 300  NEXT
 310  COLOR= 13
 320  HLIN 0,39 AT 14
 330  FOR I = 0 TO 7
 340  POKE A + I,0
 345  POKE B + I,0
 350  NEXT
 360  COLOR= 1
 370  HLIN 0,39 AT 13
 380  VTAB 21
 390  PRINT  TAB( 16)"RAINBOW"
 400  PRINT : PRINT "MIXED GRAPHICS (HI-RES/COLOR)"
 500  CALL 768

I've re-factorized the timing code in OpenEmulator, and the rainbow is
showing. My problem is the "special" lines are flickering at the
beginning.

I have checked the code, and it seems it should be flickering (the
loop waits for a non-zero byte on the floating bus), but could
somebody with a real Apple II or II plus check?

Thanks a lot,

Marc.-