Why use colourway when colour or colours does the same job with fewer words?
Because a colour scheme or colourway can include more than one colour, and the same product can be offered in multiple schemes/ways which include any given colour. If I offer a bike in metallic red fade with silver stars, or matte red with black bands, “colour” doesn’t really cut it.
I mean, we can and often do get by with “colour” even in these situations, and that’s fine. But it’s a bit like asking why we use “MPV” when “car” does the same job with fewer syllables: as words they’re just tools to do similar but slightly different jobs.
You don’t ask why we don’t use “hue” instead of “colour”, given that it has fewer syllables still. It’s because it means a different thing—certainly in the context of actually discussing colour rather than, say, acting as a loose synonym for the purposes of a cryptic crossword clue. Like colour and colourway, they refer to related things, but they’re not the same.