The simultaneous emission of red and green light with high brightness and color purity was obtained from Er³⁺-doped NaYbF₄-based up-conversion nanoparticles excited by 980 and 1550 nm excitation. The ²F_5/2 level of Yb³⁺ showed high absorption efficiency at 980 nm. The ⁴I_13/2 level of Er³⁺, an excellent UC intermediate with high energy and a long lifetime (milliseconds), absorbed more energy in the cross section (6.0 × 10⁻²⁰ cm²) than did the Yb^{3+ 2}F_5/2 level (1.2 × 10⁻²⁰ cm²) and was efficiently and directly pumped by light with a wavelength of ∼1500 nm. In contrast to particles resulting from other methods using complex coating for achieving multi-color emission in a single sample, the as-prepared luminescent NaYbF₄:25% Er³⁺ up-conversion nanoparticles were designed to be single-layered and spherical and showed excellent dispersibility and uniform sizes. Nanoparticles prepared by this method exhibit a great advantage based on the simple preparation process and small particle size. On this basis, we expect to easily achieve the emission of three primary colors by these reasonably designed core–shell particles.