Evolution. Yes the first generation of them would be black and white kind of tuxedo mix that was presented to us, but I believe that over time their descendants would gradually differ in appearance.

One being the light fury side of their looks, the other the dark fury side. Granted this would take time, as (if we assume they aren’t inbreeding within the first few generations) it would take awhile before the night lights become a fully fledged species. Then and only then can we include factors like natural selection.

You see, the night fury developed its dark scales to blend into the dark sky. The light fury to blend into the clouded sky. And while, yes, they would be living in the hidden world, that place is still a dark environment, meaning night lights with darker scales would have an easier time hiding from threats (e.g. hostile dragons, because not everything is perfect)

Therefore; I believe that overtime (perhaps by the time the nine realms occured…anyway) night lights would have slowly drifted into a species that is more similar to the night fury in appearance. With darker scales. Though at the same time most night lights would more than likely have the physical traits of the light fury, such as a smooth frill/plane thingy instead of spikes and longer legs.