Top panels: MS fiducial in 3 X − I CMDs (X = U,B,V) of NGC 6121. The horizontal lines represent the magnitudes I_CUT for which the color distance between the two MSs is calculated. For each I_CUT the error in Δ(X − I) is shown. Bottom panel: X − I color distance between rMS and bMS at I_CUT = 17.7 as a function of the central wavelength of the X filter. Observations are represented with gray dots. Red asterisks are the best-fitting model, while blue squares are the results obtained calculating synthetic colors of two MS stars with the same light-element chemical composition, but different He content. The blue squares demonstrate that the abundance of light elements assumed in the model does not affect the results of the He abundance of the two MSs in the optical colors, but strongly affects the U band.