Colorimeters (AKA color readers or color matchers) are devices that measure the lens' value for a color, which facilitates color matching for touch-ups or extensions. The human eye can be deceived by any number of contextual factors to misread a color. What kind of light is in the room, how enlarged your students are, what colors are nearby, and even what kind of mood you are in can affect how you see a certain hue at a certain moment.
Since there are thousands or even millions of colors used in commercial products such as paints and plastics, no person can reliably identify the exact brand for each shade. Color readers take the guesswork out of this process by measuring with mechanical precision the absorption of different wavelengths of light created by the current color.
Most color meters are dedicated units with their own light source. To get a read, apply a reader directly to a painted surface or to which area you want to identify the color for. Some of these gadgets communicate with an app on your phone, and some even recommend the most popular commercial color brands that are close or perfect matches.
If you want to add a color coating to a surface, or paint a nearby area, but you do not know what type of color was used for the first round, use a colorimeter to get the maximum possible precision for your project.