Question

Why are different wavelengths used for red and blue dye in a spectrophotometer?

Answer 1

You want to be able to set the spectrometer to absorb at different wavelengths. Most of the light will be transmitted through blue dye if you set the spectrophotometer on the blue wavelength and the same for red dye with the spectrophotometer set on red wavelengths. Generally, the spectrophotometer is set to a wavelength that is the complement of the color you are measuring.