Why do colorimeters have colour filters?

 

It has finally occurred to me why a colorimeter has colour filters. Last year I was using copper sulfate solution of different concentrations. A colorimeter shines light through a sample of it placed in a cuvette. It measures the intensity of the light going in and the intensity of the light coming out having been transmitted through the sample. The intensity of the light coming out will be less than the intensity going in because some of the light will have been absorbed. Copper sulfate is blue. That means it absorbs the other colours but all blue light goes through it. Blue is transmitted; it cannot be absorbed by copper sulfate. If we shine white light through, it contains blue as part of its spectrum. That blue light goes straight through. But suppose we use a filter before the sample cuvette that absorbs all of the blue light before it reaches the cuvette. Only the other colours will reach the cuvette and they stand a chance of being absorbed. This time the intensity coming out will be lower. The bigger the difference between intensity in and intensity out, the higher the absorbance. Using the correct filter increases the absorbance.