I hate to disagree here, but the ST's RGB are most definately analog (in ALL resolutions). The RGB lines output a voltage representing the intensity of a given color, between 0 and 1 volt, with 1v being the most intense (white is 1v on all lines). You can verify this by hooking a scope up to one of the lines and capturing some data. Incidentally, the VGA spec is 0.7v max, which is why you have to add a resistor on the R,G & B lines when you make a VGA adapter for the STs.
Back in the dark ages (before VGA) there were digital video setups, where you'd output binary for a given color, but that is pre-ST. (and a horrible cludge it was...)
The ST's colors are limited by screen memory and the SHIFTER chip, NOT the signal being outout to the monitor. In fact, there used to be a hack to get 32k colors by soldering up a second SHIFTER chip.
Really!
-Ken