well i think its easiest to break it down into several chunks. the most fun for me would be the printhead color mixer program. i would put it in a PICmicro or Basic Stamp2. It would not be that precise at first, but fairly easy to start. i would likely make it a 1-wire network or serial device that recieves a command like:
;make green - gantry moving 180inches/minute
cyan on 2.5us/motion speed : spray pressure 45%
yellow on 2.5us/motion speed : spray pressre 45%
so the transmission from the host to the software on the printhead would look like :
c;2.5;45:y;2.5;45
ff
but just an 8bit word or something... i dunno that much about it - i use PICbasic, it would be easy with quickbasic. to calibrate colors would also be fairly fun