V number (Abbe Number if visible)

To characterize how an imaging element changes focus with color, need to understand dispersion in terms of the "V number".

Mirror

A mirror has a constant optical reflective power versus wavlength. Therefore for a mirror the V number is 0

Refractive lens

In general the V number of a lens is defined as:


The typical equation for visible is defined as the "Abbe Number"


Here is a relative comparison of a large and low dispersion visible prism:

Reference:
Modern Optical Engineering By W.J. Smith 3rd edition

Diffractive element
The V number of a diffractive or holographic element is not at all dependent on the material properties of the imaging device. Rather it is only a function of the wavelengths of light used in it.


Reference
Diffractive Optics by D.C. O'shea, T. J. Suleski, A.D. Kathman, D. W. Parther

Here is a quick comparison of an axial color plot for a refractive and a diffracitve element