I use a rack for rolling purposes, mainly for using the bike as a shopping trolley. It adds a bit of weight and Bromptons aren't particularly light in the first place but so far I think it's worth it. I can't see I'd ever put anything on the rack, in fact I lost a bungee cord and just took the other one off.

As for colours I don't think many of the potential mixes complement each other and most look downright ugly. Having said that a coloured main frame and black extremities tends to look good IMO and sort of highlights the contruction/character of the bike, and a cobalt blue frame complements ti extremities very nicely. I think it's easier to get spare forks/stem in black, red and raw than other colours.