Just an idea: build a Wheatstone-bridge with two resistors and two photoresistos with red filter.Use for photoresistors two boxes with common lighting. Place the etalon under first photores. The bridge balanced only, when You place under another photores. the same color.
Just be sure that you understand the sensitivity of the light sensor to the colors of interest. Many sensors are very broad but some aren't and could be "blind" in some parts of the visible spectrum.