The color of an object depends on the temperature of the object. This phenomenon is known as the thermochromic effect, and it is already noticeable at room temperature if the temperature varies just a few centigrades. Red and orange samples are especially sensitive to temperature variation and may cause difficulties in precise color measurement. In this article we show how the phenomenon is based on physical processes and does not only reflect the instability of red color pigments. We derive simple formulas that are shown to be able to explain the published experimental data.