True but irrelevant. The style of the medium coevolved with those constraints and is suited towards them (as with most media). A manga-style drawing is stylised in particular ways that (among other things) allow it to look better in black and white than a typical drawing would.
> There are tons of full-color manga, and many that become popular have color editions released.
They exist (indeed I own some) but they are the exception rather than the rule. Again, "manga-style drawing" would be generally understood to mean a drawing that looked like a typical drawing from a manga.
Generally understood by who though?
I see your point about what you said about the colours
> manga-style drawing is stylised in particular ways that (among other things) allow it to look better in black and white than a typical drawing would.
In particular I agree that the black and white manga drawing uses special techniques to look good in black and white. Such as notably the half-tone patterns that they use.
But I think you are placing more weight on this particular thing than perhaps most people do.
There are many defining features of manga aside from the lack of colour.
I guess to determine whether or not it is generally agreed that it must be black and white, we’d have to interview a bunch of people about what style some images were, and see if an overwhelming majority thought that colour mattered or not :p
Or, less thoroughly we might just ask the denizens of the 4chan /a/ board. They probably have some strong opinions about what counts as manga or not.