A strategy is demonstrated for simultaneous size and color tuning of KZnF3:Yb3+/Er3+ nanocrystals through transition metal Mn2+ doping. The experimental results indicate that the introduction of Mn2+ into the KZnF3:Yb3+/Er3+ reaction system facilitates the decrease of the crystal size. Moreover, in the case of a high concentration of Mn2+ doping, the downshifting (DS) and upconversion (UC) luminescence all show a single red band emission. In this work, we mainly investigate the DS and UC energy transfer process among Mn2+/Yb3+/Er3+ in KZnF3 nanocrystals by the detection and analysis of the absorption, excitation, emission, and transient fluorescence spectra. The results show that both Mn2+ and Yb3+ ions can decrease the green emission while improving the red luminescence of Er3+. This DS and UC single red color tuning is significant for the application of nanoparticles in light conversion, biological labeling, or plant photosynthesis.