[Fe(CN)6]4– and [Fe(H2O)6]2+ are of different colours in dilute solutions. Why?

The colour of the particular complex compound depends on the crystal field splitting energy(CFSE). This CFSE depends on the nature of the ligand attached to the metal atom. In case of  [Fe(CN)6]4–  and [Fe(H2O)6]2+ the colour differs due to differences in CFSE . CN- is a strong field ligand so will have high CFSE than H2O with a low value of CFSE. There is absorption of the energy from the visible region for the d-d transition and corresponding complimentary colour is observed. Thus there is the colour difference.