Critical angle:

$$\sin\theta_c = \frac{\mu_{rarer}}{\mu_{denser}}$$

Since $\mu$ depends on wavelength (colour) of light — violet has higher $\mu$, red has lower $\mu$ — the ratio $\mu_{rarer}/\mu_{denser}$ is different for each colour.

Therefore, $\theta_c$ is different for different colours:

• Violet light: highest $\mu$ $\Rightarrow$ smallest $\theta_c$
• Red light: lowest $\mu$ $\Rightarrow$ largest $\theta_c$

This is why TIR can occur for some colours but not others at the same angle.