I imagine you have all you need to have the job done if you use JMRI to pilot a DCC++ base station and have build a monitor : you can watch what JMRI send on the tracks when commanding RGB LEDs. Perhaps there something in the Help. (there is a video on youtube - where ?) and then adapt the decoder to interpret the accessory codes. Certainly easier to say than to program it. Did you try google to see if anyone has already worked on this ?