Would it be practical to instead multiplex the DMX input to the controller between it and the Arduino and when you get the command for the color you are looking for, insert the different command in from the Arduino to the controller. That way you wouldn't have to mess with recreating those analog bits. Or maybe I don't understand the problem.
Here's reference for the full picture:
the global DMX controller (front of the house) is daisy-chained to 6-10 LED controllers (back of the house) one on each lane running pit lights creating patters that beat to the music during glow bowl. When a strike is made (event trigger - from pinsetter) this prototype has to interrupt the global scene only running on that one lane for a visual effect (i.e. flash between red and white for 3-5 seconds) then allow the pit light to return to normal operation. All without interrupting the running global scene.
the problem with putting the "strike-sense" in the DMX line is that there would need to be dip-switches to inform the code of which signals to override.
now thinking of it, if the MC can sense all (say) 8 strike sensors. and only interrupt the DMX on the three specific channels. the only problem I see with this is can the Arduino run this all in parallel. It can't hesitate.
**pardon my brain storming. there is no one to bounce ideas off of here in this part of Florida**
here is what we (2 person team) have done so far at one of our centers- http://youtu.be/k6zvsgURmrE
note: all this was installed/programmed in-house by us at two of the four of our centers we are hoping with all this R/D we can get our own department... an office... something.