IMHO ano stuff only looks good in moderation, on the right coloured frame and in balance. For example, a set of hubs (mate has a Canyon with a light putty coloured frame and lilac decals, purple hubs really pop on it). A large single coloured item like a stem on an otherwise all black component bike will just look out of place, it needs a seatpost collar to balance it (and even then most times a stem in any colour but black or silver looks daft).
So for me, to answer the OP, black, or if you go for a colour silver and a silver seatpost collar for balance.