Theoretical values are determined for the color indices U-B, B-V, V-R, and R-I of the radiation of a plane layer of hot hydrogen gas at temperatures from 10,000 to 100,000K and for various optical thicknesses. Two cases are examined: an isothermal medium and a layer, one surface of which is heated by an external energy flux. Diffuse radiation of the layer is taken into account. Tables and diagrams of the color indices are given. Possible applications to flare stars and quasars are discussed.