The focal length of a lens is given by the lens maker's formula

= (μ-1){ - }

Clearly, the focal length increases as μ decreases. μ depends on the color of the light. So the focal length of a lens depends on the color of the light.

Since there is no refraction in the mirror and it works on the laws of reflection which is the same for all colors. For example, for the same angle of incidence, the angle of reflection will be the same for all colors. Hence the focal length of a mirror does not depend on the color of the light.