Colour of ${\mathrm{KMnO}}_4,{\mathrm{K}}_2{\mathrm{CrO}}_4,{\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7$ is due to

 The number of unpaired electrons in $\mathrm{K}\stackrel{+7}{\mathrm{Mn}}{\mathrm{O}}_4,{\mathrm{K}}_2\stackrel{+6}{\mathrm{Cr}}{\mathrm{O}}_4,{\mathrm{K}}_2{\stackrel{+6}{\mathrm{Cr}}}_2{\mathrm{O}}_7$ respectively are 0,0,0.

Un paired electrons are not available, hence they should be color less, but they exhibit color property due to charge transfer between ligands and central metal.