The equilibrium constant for the formation of CoCl42-, a species that is blue in solution, is to be — Laboratory Chemistry Question

The equilibrium constant for the formation of CoCl^42-, a species that is blue in solution, is to be measured using a colorimeter. The colorimeter has wavelength settings of 470 nm (“blue”), 565 nm (“green”), and 635 nm (“red”) to use in the experiment. What is the best setting to use?