Specific color sensing, is it possible in an economical way, without using camera/image recognition?
Since LEDs used as photodiodes are most sensitive to the colour they emit during normal operation, a basic colour sensor can be made using the LED in reverse and an opamp:  



The circuit above came from this page. It can also be done the other way round - a much more detailed look at colour sensing using LEDs and light sensors is available here - this page details using a normal light sensor and different colour LEDs.

I couldn't find the app note mentioned in the comments, but this page seems to be quite a thorough treatment of the subject of transimpedance amplifiers. You can alter the bias across the LED to change response time/sensitivity.