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.