well yeah, but the real stuff (NB1) is very fugitive and I've only seen it in dry pigment form. There's apparently a synthetic version (PB66) from a few brands like mussini (dickblick link
), but according to dickblick it sounds like it's also fugitive, or at least something I wouldn't trust. Since indigo is a purplish blackish dullish blue I think a really good replacement would be indanthrone blue (PB60) mixed with a touch of black.
The problem with making a painting like starry night by van gogh (as you mentioned in your PM) and making it look real is that it's like those are two different things, at different ends of the real/not real spectrum. His night sky, while very good and likable, is also extremely not real looking, and if you were to make it look real then it'd no longer look like starry night. Does this make sense?
Now if you just want to make a night sky and not have a specific painting in mind to make it be like then there's a ton of questions to consider. Is it early in the night, the dead of night, nearing morning? Is it in the city where the stars disappear or in the country? Are the stars a major focus of the image? Is it a full moon? Are there clouds?
The second to the last painting here
is a good example of a realistic night sky at the end of twilight when the only light left is a faint blue glow on the horizon.