average color to RGB sensor?

I have a customer with 3 lit ceilings... They asked me if it was possible to capture the average color of the sky and translate it to RGB information. With video I/O and a more expensive controller this would be easy, with a small unit and essentially 3 huge fixtures it is more difficult. 

Has anyone done this? In some quick google searching I found that I would want a 0 reference as well as a sensor with some sort of filter for violet light. Otherwise, not sure where to start. I like the concept so it is worth exploring. Maybe using an arduino/RPi with cheap sensor is the answer, perhaps someone knows of a more elegant solution?

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