Colorimetry is not a difficult concept to grasp. A four-year-old child understands basic colorimetry. Let’s say I make two glasses of red juice for this child using a powdered drink mix. First, I fill both glasses to equal volumes of water. Then, in the first glass I add only one tablespoon of drink mix. In the second glass I add two tablespoons of drink mix. Once mixed, I serve both drinks to the child. The child sips both and rejects the first glass because the juice isn’t as sweet as in the second. I guess I should have seen that coming because, after all, I only put one tablespoon of drink mix when the recipe called for two. Not everyone enjoys drinks made by someone else.
But to say the juice is more sweet or more red in one glass than in the other still leaves a lot of room for error in the judgement of the taster or observer. What if the differences in drink mix concentrations had been only 1 teaspoon instead of one tablespoon? What if the difference had only been 1/8th of a teaspoon?
And what if I wanted to measure the concentration of something like a blue window cleaning solution? I certainly wouldn’t want to taste it to figure that out.
This is the essential problem addressed by colorimetry and Beer’s Law: how does one quantify even the most subtle of differences in the concentration of two solutions? Answer: with color reactions, light, and an instrument for measuring light.