Yb3+/Er3+ codoped yttrium aluminium garnet (YAG) porous nanocrystals were prepared by glycolate method assisted with poly-vinyl alcohol (PVA) and urea. The typical cubic structure for YAG was confirmed from XRD with crystallite average size of ~40 nm, calculated from Scherrer formula and corroborated by TEM. Strong green and red upconversion emissions are observed readily with the naked eyes, and the color coordinates were obtained from emission spectra. A theoretical model to calculate CIE coordinate as a function of donor (Yb3+) and acceptors (Er3+) concentration is proposed. The eye-safe near infrared emitted signal and fluorescence lifetime were also measured and results show lifetime as large as 8.5 ms. The maximum energy transfer efficiency from Yb3+ to Er3+ was 72% for 20 mol% of Yb2O3. The proposed mechanisms for signal emitted are explained in terms of direct and energy back transfer processes, and cross relaxation.