The pale colour of ${\left[\mathrm{Mn}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{2+}$ is due to

In ${\left[Mn{\left(H_{2}O\right)}_6\right]}^{2+},Mn$ is in +2 oxidation state with ${\text{d}}^{\text{5}}$ configuration and  $t_{2g}^3$ and $e_g^2$ electron distribution. Since all the d orbitals are filled with one electron each the d-d electron transition is spin forbidden.