Looks good so far.
Of course, all the detail will be in "Change voltage counts into PWM steps".
0-511 will control brightness of one color, 512-1023 will control brightness of the other? (keeping in mind that analogWrite() only takes 0-255 as a value)
Maybe something different so both can be on? Maybe 0-255 is only one fading up, 255-511 is onlythe other fading up, 512- 755 is both fading up, 756-1023 is both fading down?
Lots of options.