Indeed, it’s complicated.
I use a Tonino Colour Analyser post roast which gives me the roast depth as per the Agtron scale. But its only a benchmark.
A Light roasted bean here in Austrlia would generatlly be considered Medium in Seattle. And what the Tonino tells me is Medium looks dark to me, complete with the tell tale sign of dark roasts which is oil on the outside of the beans.
And as DC points out, you may have two beans that appear to be the same color and yet one of them will taste light and fruity and the other my be all chocolate and toffee due to either the roasting recipe or the type of bean (smaller high altitude versus bigger lower altitude), or both.
That said, I still label each batch according to my own judgement and it only really becomes an issue if I am comparing notes with someone else. For example a friend says that they poured a medium roasted bean in a certain way and that I should try the same receipe, may be referring to a roast depth that I would describe as dark. Lost in translation.
Like I said, it’s complicated. But mostly only when communicating with others. If you roast your own beans it’s not a biggie and if you are buying from the same roaster then you’ll soon be able to translate what they call a dark roast into whatever label you would have put on it.