AbstractThe transformation of the blue color of bentonite‐benzidine pastes to green or yellow‐green upon air drying was studied by means of reflectance measurements and by comparison of the samples under study with color charts. The relative amounts of benzidine and bentonite and the relative humidity had a decisive effect on the color of the air dried films. Hygroscopic compounds such as LiCl and CaCl2 produced results similar to that produced by increasing the relative humidity. The explanation of the color reactions is based on the assumption that the (blue) monovalent and the (yellow) divalent cations coexist in the air dry benzidine‐bentonite films.