Black is the most obvious choice, and tends to work the best with more formal ensembles. It does have two disadvantages, though; it is the default color for many manufacturers, and for this reason can look a tad generic (this, of course, is relative) and sometimes make it look as if the wearer is trying too hard by focusing on the item rather than how it suits him.
If black slippers are chosen, some sort of subtle emblem is generally desirable. I have a red-lined pair from Ralph Lauren (C&J, I believe) in black velvet with a black "RLP" insignia, and the tonal embroidery gives them more depth than a plain toe or an ornate gold crest. They came to me secondhand, however, and I'd have preferred for the ornamentation to have been of some personal relevance. It's important, when wearing a potentially pretentious item, to keep it defensible and avoid motifs that relate to sports you don't play, schools you don't attend, etc.
Navy blue is a good choice for velvet, and can be worn unadorned or otherwise. It goes with a wide range of items while being arguably less conspicuous than black or some brighter color.
Finally, for summer, needlepoint is the most appropriate. I've never cared much for linen slippers, as I find them somehow off-putting in all but a few cases. The material seems to be at odds with the shoe's style. It takes balls to wear needlepoint slippers, but if you've got them and can treat them as you would any other shoe they work splendidly. Take a page out of this guy's book and keep the rest of the outfit subdued, so as to avoid looking like your significant other is making you wear them while you're on vacation.
That pair is from Stubbs and Wootton, and I hope to obtain a pair at some point.
They're among the better needlepoint offerings I've seen.
-VF
