via Wikipedia:

The Standard Reference Method or SRM [1] is one of several systems modern brewers use to specify beer color. Determination of the SRM value involves measuring the attenuation of light of a particular wavelength (430 nm) in passing through 1 cm of the beer, expressing the attenuation as an absorption and scaling the absorption by a constant (12.7 for SRM; 25 for EBC). The SRM (or EBC) number represents a single point in the absorption spectrum of beer. As such it cannot convey full color information which would require 81 points. But it does remarkably well in this regard (it conveys 92% of spectral information) even when fruit beers are considered. Auxiliary “deviation coefficients” (see Augmented SRM below) can pick up the remainder and are necessary for fruit beers and when subtle color differences in malt beers are to be characterized.

The SRM is something I’ve began to use in my beer reviews. of course, for most casual reviewers like me, this model is a big guess. We eye-ball the beer a few times, lookup the SRM chart and then try to give our best 1-80 number on the beer’s appearance. I still do a basic appearance description but SRM is something I’ll add and hopefully get better at determining automatically the more I do it.

I think this is a good way to review beers and not one I ever see in beer reviews. I’m sure this is because it’s hard to be exactly. Of course your pale ale will get a 2-4 on the scale and the reader has no idea what you’re talking about without looking at the scale and, even then, it’s not really specific as the SRM scale is a form of machine tests with a lot of math. Doing an SRM analysis on every beer you drink is impossible so most of us don’t use it.

I think it’s fun and here are a few links to read: