Nonlinear optical properties of NaYF4 nanocrystals (NCs) doped with Er3+ and Yb3+ ions are investigated. The upconverted luminescence spectra show blue, green and red color emissions from the NaYF4: Er3+/Yb3+ NCs. The color output can be tuned with Yb3+ ion concentration, which is due to the modifications in excited-state absorption and energy transfer process between the Er3+ and Yb3+ ions. The increase of Yb3+ ion concentration in NaYF4: Er3+/Yb3+ NCs causes accompanied increase of nonlinear absorption, as well as nonlinear refractive index, values. The generated electrons with increased Yb3+ concentration in the samples lead to the increased nonlinearities from enhanced multi-photon absorption process. (C) 2010 Elsevier B.V. All rights reserved.