1) YES! I really do like this finish. Speakers like this are clearly geared towards matching or accenting the style of a room, and this is a great option.
2) I like the colors you already offer, and I think it would be great to continue the line with other neutral colors. Perhaps some variation of a sage green, some other browns both lighter and darker, and smoke gray would be a good start. I'm not sure how difficult it is to come out with different colors, but you could certainly offset the cost for some of them by charging a small "custom color" fee, and perhaps have them built to order so you don't have to maintain stock of the less popular ones.
3) Subs are best in black or darker wood colors. That's my opinion. I know some people would like them to match the speakers, but I've always wanted the sub to be out of the way and not attract attention.