The absorption spectra of [ Cr^III  (H ₂O) ₆] ³⁺ shows a peak at 17830 cm^-1 at which the red and violet light are absorbed and thus the transmitted light is the complementary colour yellow-green. That is why it appears green.

[Fe^II  (H₂ O) ₆] ²⁺ shows the absorption peak at 10400 cm^-1 which does not fall in the visible range , hence colourless.

[Fe^III  (H ₂O) ₆] ³⁺ shows the peak at 13700 cm  at which the colour absorbed is blue-green and the transmitted light is red.

[Cr(H2O)6]3+ undergoes d2sp3  hybridisation.there r 3 free electrons which contribute for d-d transition.

there r no free electrons in [Fe(H2O)6]2+