What instrument is used to measure the amount of chemical in a solution based on its color?
A spectrophotometer passes a beam of light through a solution. Some light passes through the solution, some light is absorbed. A detector can measure the transmittance, how much light passes through the sample. This allows you to calculate the amount of light that passed through the sample.
It is possible to use different wavelengths of light since a solution of a compound will absorb some colors better than other colors.
The video shows a spectrophotometer set to 625nm being used to measure the amount of
Hope this helps!