The essential parts of a colorimeter are:
* a light source (often an ordinary low-voltage filament lamp);
* an adjustable aperture;
* a set of colored filters;
* a cuvette to hold the working solution;
* a detector (usually a photoresistor) to measure the transmitted light;
* a meter to display the output from the detector.
In addition, there may be:
* a voltage regulator, to protect the instrument from fluctuations in mains voltage;
* a second light path, cuvette and detector.  This enables comparison between the working solution and a "blank", consisting of pure solvent, to improve accuracy.
There are many commercialized colorimeters as well as open source versions with construction documentation for education and for research.