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