This paper discusses a method to measure the thickness of thin layers deposited on a reflective substrate. An interferometer with three wavelengths produces color interferences. A color sensor records the tint that is produced. The color interferences are approximated by a model based on the measurement of the laser intensities obtained with the reference mirror only. An iterative process leads to unambiguous algorithmic convergence and high accuracy thickness measurement. This method is simple, robust, compact, and single shot. The method does not need angular scanning over the field of measurement (about 75 mm2). The measurement on the surface yields a histogram of the thickness distribution and there is no requirement for any reference points (e.g., no need to make a groove or a walk on the layer). A thickness measurement performance of 50 nm was demonstrated for homogeneous polymer films deposited on a silicon wafer. The setup and digital image processing are discussed.