A single-scattering formulation of the color index is presented to investigate the physical mechanisms governing the CI maximum observed during twilight in the presence of high-altitude clouds. Within this framework, the maximum is shown to occur at the solar zenith angle (SZA) for which the relative SZA-variations of the zenith intensity become equal at the two selected wavelengths. The formulation is validated against Monte Carlo radiative transfer simulations, showing good agreement.