The beer is red. No, amber. No, caramel. 

If you really want to know the colour of beer you could use a spectrometer. The Spectrometer would precisely control light to select a single wavelength from the spectrum of visible light (about 400-700nm) and pass it through a sample and measure the degree of transmission or absorbance.

But we would probably drink the beer before I found a spectrometer. So…..

You say tomato, I say tamaeto. But we can both agree on a colour chart.

The Standard Reference Method (SRM), developed in the 1950’s approximates the colour of finished beer using a visual method.