The octahedral complex of a metal ion  ${\mathbf{M}}^{3+}$with four monodentateligans ${\mathrm{L}}_1,{\mathrm{L}}_2,{\mathrm{L}}_3\text{ and }{\mathrm{L}}_4$absorb wavelengths  
in the region of red, green, yellow and blue, respectively.  The increasing order of ligand strength of the four ligands is 

For a given metal ion, weak field ligands create a complex with smaller $\mathrm{\Delta }$ which will absorbs light of longer $\lambda$ and thus lower frequency. Conversely, stronger field ligands create a larger $∆$absorb light of shorter  $\lambda$and thus higher frequency i.e. higher energy.