Colloidal nanocrystal layers deposited onto the enclosure of InGaN light emitting diodes are demonstrated to operate as nano-phosphors for colour conversion with high colour stability. Depending on the choice of the nanocrystal material (either CdSe/ZnS or PbS nanocrystals are applied), the diode emission at 470 nm is converted to red or to infrared light, with similar quantum efficiencies. The colour conversion is further improved by dielectric mirrors with high reflectivity at the emission band of the nanocrystals, resulting in an almost doubling of the nanocrystal light extraction from the devices, which increases the nanocrystal device efficiency up to 19.1%.