Beer colour and colour units

I'm currently reading the Malt book in the Brewing Elements series. I was pleased to see a reference about beer colour from Breiss malt I remembered reading, but had forgotten where.

A powerpoint presentation explains how a range of different coloured beers can get the same value when the colour is measured in a laboratory. As different malts absorb light in different parts of the spectrum we get beers of different colour, but laboratory measurement at 430nm can't always tell them apart. Dark roasted malts absorb more strongly across the spectrum, leading to darker beers and browner tones at equivalent colour value.

So much like bitterness units are a very incomplete picture as a flavour descriptor, colour units don't actually tell you what colour a beer is.

You really need to look at the slides, but here's a little taster which I've translated into English:

Colour (EBC)	Caramel malt	Black malt
20	Orange	Tan
40	Red	Brown
60	Mahogany	Black