The color of beer is an interesting topic. Beer color is measured on an SRM scale, which stands for “Standard Reference Method.” SRM is measured by passing a beam of light (at 430 nm wavelength) through 0.39 inch (1 cm) of beer and measuring attenuation of the light. The 430 nm light used corresponds roughly to a blue-violet light.
The problem with measuring light attenuation at a single point is that it does not convey all the possible color information that comes from full-spectrum light passing through beer. In fact, it would take some eighty-one measurement points to properly characterize the color. We like to think of beer color being limited to the SRM linear color scale, but in reality it’s more like a color wheel that may vary toward any of the basic colors. However, the SRM method does a remarkably good job of characterizing more than 90 percent of the color information in an average beer.
These limitations become obvious in fruit beers or beers such as the Irish red, where colors outside the normal yellow-brown/dark-brown tones come into play. The one-dimensional SRM scale simply does not work well for capturing the deep blue of a blueberry beer, red from raspberries, or even the red tint in an Irish red.
Consider also that relatively few people have access to a photometer or spectrophotometer to measure SRM directly. Instead, they must rely on estimates calculated from an equation, along with color data that the maltster provides. Both of these potentially introduce additional errors.
While the malt provider will usually publish a data sheet with their malts, there is some variation in color from batch to batch as the malts are individually kilned, dried, and stored. For beer-color estimation, the Morey equation is most widely used, but it is nothing more than a curve fit to experimental data taken under laboratory conditions that may or may not match your actual brewing conditions. Even small things, such as slight variations in batch volume and temperature, can have an impact on the final color of the beer.
All of that being said, the Morey equation does give us a reasonably good estimate of beer color, but as with all estimates, it is not going to be perfect. Similarly, the SRM color scale, which attempts to compress what is really a color wheel into a straight line, has its limitations, but is still a good starting point for the average brewer. Only by brewing do we get to appreciate the full spectrum of light going through the glass!