vaguely related - in the old days there was a similar technique to get extra colours out of a screen. This was on the Amstrad but applied to other systems to - there was a 16 colour lookup table that you could write to which was the 16 colours displayed on the screen. You could get an interrupt on horizontal retrace and so at the start of each horizontal retrace you could switch the colours in the table to give you extra colours. You had to be quick though you had the time it takes for the electron beam to get from the right hand side to the left hand side of the screen.