Build an oscillator with the LDR in the freq determining portion.
Send the output to a freq to voltage convertor and read the output on a voltmeter(analog).
AFTER calibrating, you can make a new backing plate for the meter and put the colors on it. The meter will 'point' to the color based on the output of your freq to voltage circuit.
That's the best I can offer idea wise, that remains grade school simple and easy to understand.