The colour of  $K_{2}C{r}_2{O}_7$  is due to

$L\to M$ Charge transfer spectra $K_{2}C{r}_2{O}_7$.  is coloured because it absorbs light in the visible range of electromagnetic radiation. The permanganate ion is the source of colour as a ligand to metal   ($L\to M$ ) charge transfer takes place between oxygen’s ‘p’-orbital and the empty d-orbital on the metal. This charge transfer takes place when a photon of light is absorbed which leads to the Orange colour of the compound.