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 Yb(3+)/Er(3+), Yb(3+)/Tm(3+) and Yb(3+)/Ho(3+) doped Na3ScF6 can be tuned by altering the Yb(3+) 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:Yb(3+)/Ln(3+) shell on Na3ScF6:Yb(3+)/Ln(3+) cores with respect to that of pure Na3ScF6:Yb(3+)/Ln(3+) 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.