"rarely has idle CPU" ? I'm guessing that "idle CPU" is supposed to mean the system is booted up but has nothing to do at the moment, so CPU is powered down as much as possible. Then the question is about finding something for RPi to do at that time.

If the RPi has the chops for your hardware problem, you can do it with experix. This is an interpreted language system that provides all the calculations you might want to do, easy-to-use graph drawing, conditional branching, timers, usb, webcam and socket operations, and (of interest to you) abilit...

"color scanning with a USB camera" seems to be exactly what I have demonstrated (among other things) in the 'pictures' file in experix_r14_book.tgz2 from experix.sourceforge.net (freely available open-source project recently adapted to run on raspberry-pi). Please have a look at this, and contact me...

I can view store.raspberrypi.com but clicking any link on that page makes my browser spin for 10 seconds or so and then show me a completely blank page.