The black ones, that I've seen and handled, were near-perfect replicas of the original vases - the weight and thickness of the glass was pretty much spot-on. The only thing that made me think that they were likely not authentic was the colour.
Then there's not forgetting the Gulvvaser that were made in England under licence from Holmegaard in something very like this light blue colour - so far I've only seen them in 10" and 12", but that does not mean that larger ones don't exist. These too are identical to the originals, except for colour.
I think a lot of the copies made in mainland Europe washed up in the UK - Denmark is probably quite lucky that way. Conversely, you probably see lot more of the genuine oddities, and are more open-minded towards them.
So I'm not saying that undocumented colours do not exist, as I've owned an Olivengr√łn/Opal vase myself, and have seen one using Kastrup's Lila/Opal colour-scheme, which was mostly used for glass lamps. But I think these are the exceptions rather than the rule - especially outside of Denmark.