The colour of potassium dichromate is due to

Theorange color of dichromate is due to charge (electron) transfer from ${\mathrm{O}}^{2-}$ ligands to the empty d-orbitals of the central metal $\mathrm{C}{\mathrm{r}}^{6+}$ , i.e. LMCT. This electronic transfer absorbs greenish blue light and leaves the complimentary orange light behind as the observed colour.