The octahedral complex of a metal ion ${\mathrm{M}}^{3+}$ with four monodentate ligands ${\mathrm{L}}_1,{\mathrm{L}}_2,{\mathrm{L}}_3$ 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

Coloured regions of visible light (VIBGYOR) are arranged in the increasing order of wavelength or decreasing order of energy $\left[\because \mathrm{E}\propto 1/\mathrm{\lambda }\right]$. Therefore,

$\underset{\left({\mathrm{L}}_1\right)}{{\mathrm{E}}_{\text{Red}}}<\underset{\left({\mathrm{L}}_2\right)}{{\mathrm{E}}_{\mathrm{Yellow}}}<\underset{\left({\mathrm{L}}_3\right)}{{\mathrm{E}}_{\mathrm{Green}}}<\underset{\left({\mathrm{L}}_4\right)}{{\mathrm{E}}_{\mathrm{Blue}}}$

This suggests that $L_4$ will absorb higher energy wavelength followed by ${\mathrm{L}}_2,{\mathrm{L}}_3 \mathrm{and} {\mathrm{L}}_1$. Since, higher absorbed energy corresponds to greater extent of crystal field splitting and thus, higher field strength of the ligand.
Hence, the increasing order of ligand strength of the four ligands is

${\mathrm{L}}_1<{\mathrm{L}}_3<{\mathrm{L}}_2<{\mathrm{L}}_4$