It is to do with something called the crystal field stabilisation energy (CFSE). Cr2+ had a dn configuration of d4, Mn2+ has a dn configuration of d5. Water is a weak field ligand and thus the complexes are high spin. This means that the CFSE for the Cr2+ complex is -3/5Δo where as it is 0Δo for the Mn2+. Different Δo=different colours
Absorption of light of correct frequency promotes electrons from the ground state to an exited state and is responsible for the colour of the metal complex. This is called a d-d transfer (transition) and can be measured
by absorption spectra