[revised on Tuesday, March 20 when I got the real answer from Scott at Redding Audio:]
In the mid-1970s there definitely were silver-colored Colette cables--I still have one. It came with dark fittings or fixtures--not matte black, but a smooth, non-glare olive green. So even back then, the difference was only in the color of the cable jacket.
Frankly I have no idea any more why I got it in silver; for me, one major reason for a Colette cable's existence is its near-invisibility. Anyway, they've upgraded the cable stock several times since then, and are now using a more substantial, Kevlar-jacketed type. Like the Ford Model "T" you can have that in "any color you want, as long as it's black."
Most Schoeps microphones and many accessories are available from stock in either gray or silver; as an exception, their CMIT 5 shotgun mike is blue. However, microphones may be specially ordered in almost any color, and you could similarly order Colette cables with their metal parts finished in any color you want, including nickel. But that would incur extra cost and delivery time--and the cable itself would still be clad in black Kevlar, ready for night-time expeditions into enemy territory.
--best regards
P.S.: It's interesting to see how people interpret the "g"s in Schoeps' Web listing (which is taken from the main catalog text). They weren't meant to imply anything about finishes not being available; the intention at Schoeps is to let dealers and distributors know which version will be delivered by default if they don't specify either "g" or "ni" on an order. As you noted, mostly it's "g" nowadays. I don't think they quite imagined that customers would read other meanings into this procedure, though of course it's only natural to do so.
Actually the whole nomenclature for Schoeps products has become so baroque that most real-world users seem to ignore it; people expect complete microphones to have model names, and they expect the microphones to have a certain definite "look." But Schoeps microphones can look entirely different depending on which accessories you use with them (e.g. the active Colette tubes which have been used to record "The Three Tenors" so often). Sometimes I show up for a recording and the client will say, "I see you have new microphones" maybe for the fifth time, and I don't know whether to explain that no, I've been using the same ones all along, but they've been configured a little differently each time for whatever reason.