In this paper, well-defined and regular-shaped Na₃ScF₆ nanocrystals (NCs) have been synthesized in high boiling organic solvents 1-octadecene (ODE) and oleic acid (OA), via the thermal decomposition of rare-earth oleate precursors. It is found that highly uniform monoclinic Na₃ScF₆ NCs with narrow size distribution have been obtained, which can easily be dispersed in cyclohexane solvent to form transparent colloid solutions. Upon 980 laser diode (LD) excitation, the relative up-conversion (UC) emission intensities of different colors in Yb³⁺/Er³⁺, Yb³⁺/Tm³⁺ and Yb³⁺/Ho³⁺ doped Na₃ScF₆ can be tuned by altering the Yb³⁺ doping concentration, resulting in the tunable multicolor in a wide range. On the basis of the emission spectra and the plot of luminescence intensity to pump power, the UC mechanisms of the co-doped Na₃ScF₆ NCs were investigated in detail. Moreover, the UC emission intensities can be significantly improved by coating a layer of Na₃ScF₆:Yb³⁺/Ln³⁺ shell on Na₃ScF₆:Yb³⁺/Ln³⁺ cores with respect to that of pure Na₃ScF₆:Yb³⁺/Ln³⁺ core NCs. Furthermore, transparent and UC luminescent NCs/polydimethylsiloxane (PDMS) composites with regular dimensions were also fabricated by an in situ polymerization route. Uniform NCs with a wide variation of luminescence colors will show potential applications in diverse fields.