What word, or phrase, do chemists use to describe the color saturation of a solution?
Using a spectrometer, you could accurately measure the degree to which light is absorbed by a solution ("absorbance" or "optical density") at a certain wavelength, and in a cell of certain dimensions. So, if you had a blue dye dissolved in water (which might appear blue because it primarily absorbs orange light), the absorbance (measured using orange wavelengths of light) would be proportional to the concentration of dye in water.