The colour which has least critical angle is

$\begin{array}{l}\text { For total internal reflection to take place. } \\\qquad \begin{array}{l}\mu\sin\theta_{c}=1 \\\theta_{c}=\sin ^{-1}\left(\frac{1}{\mu}\right)\end{array}\end{array}$

$\begin{array}{l}\text { As a fact, the refractive index for a ray is inversely } \\\text { proportional to its wavelength. }\end{array}$

$\begin{array}{l}\quad \mu \times \frac{1}{\lambda} \\\text { Since violet has shortest wavelength it's critical } \\\text { angle is also low }\end{array}$