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|