Thicknesses of hexagonal boron nitride (hBN) on SiO2 were identified by the colors in the wide range from a few to hundreds of nanometers. We theoretically determined the colors of hBN on SiO2 as a function of the thickness in the standard red green blue (sRGB) color space. Theoretical RGB values of hBN with a given thickness were found to be in good agreement with those extracted from optical images, suggesting that the colors can be a standard for the thickness identification. This thickness identification method is facile, rapid, and versatile, and, therefore, would spur further researches on hBN. (C) 2019 The Japan Society of Applied Physics