OK so that is a current-output 4bit PWM DAC, 8*3 times, isn't it?
The resistor(s) are supposed to set a certain reference, I suppose. In the Toshiba parts we use now for bicolor displays, they are no PWM, just the constant-current sources (gated by the shift register-latch); and the current output is proportional to the current flowing throughthe adjustment resistor (i.e. they are simply current mirrors).

All in all, I thought, you are planning a circuit for a 16^3-color display adaptible to ambient light and LED degradation and other effects, and retaining all the color level possibilities. Combining the current-mirror construction (plus relatively slowly regulating the current into control pins based on feedback and/or preset values) and logarithmic PWM, I think, this is not impossible to achieve.

Just interested, what is the planned thermal dissipation of the chip (what kind of package)? You undoubtly considered multiple voltage power rails for the different colour LEDs...

Jan Waclawek