Photoluminescent (PL) properties of undoped nanocrystals of cadmium sulfide were investigated as a function of excitation power intensity. Room-temperature PL spectra of CdS nanocrystals grown from the gas phase revealed two emission bands: with peak positions at 510 nm (near-band-edge emission) and close to 690 nm (deep trap defects). Tunable photoluminescence of CdS nanocrystals with the exchange of the main radiative channel from relaxation through defect levels to direct near-band-edge relaxation with the change of the color was demonstrated. Nonlinear behavior of the intensities of near-band-edge and defect level emission lines as well as the blue shift of the peak of defect level emission are discussed and explained by the finite capacitance of the defect subzone in the forbidden gap. The origin of the red-light emission is due to native defects such as sulfur vacancies or twinning interfaces.