A colorimetric chemosensor uses a chemistry where something changes color depending on concentration of whatever you want to measure, and the sensor measures that color change.
Litmus paper is perhaps the most commonly known color change with a concentration [H+]; if you rigged up a digital camera and read the color change that would qualify as a colorimetric chemosensor.
Other pH indicator systems exist, and iodine + starch or sugar turns blue. There are some redox reactions with color changes as well.
I believe that almost all of the automated clinical analyzers used in medicine are colorimetrically based