I'm going to amend my earlier statement:
I have never seen a comic transition to color, and thought "I wish it was still B&W".
The main reason I prefer color is that it makes the art easier to "read". I don't have to try to ID characters by nostril width, or hope names are dropped in the dialog. I can just remember "person with red hair", "person with green skin", etc. It also distinguishes things from the background - SMBC generally has extremely dull, desaturated, dark backgrounds, while the characters wear much brighter, contrasting colors. So I can *immediately* tell what those characters are doing.
Taking a look through my "updates 3-4 times per week" folder, I see 6 out of 20 that are primarily black and white. All six of those are notable for clean, high-contrast art - XKCD is the extreme case, where the art is *extremely* "simple-looking". You can tell what the background is because it's white, while everything black is either a panel border, a character, or an important item. Or a graph joke, because who doesn't like graph jokes? The rest are still very clean, high-contrast B&Ws - Starslip, Basic Instructions, etc.
I've personally seen several comics transition from B&W to color while reading them. Dr McNinja stands out as the best example. The original art was fine. A bit rough, but fine. Then it went color, and never looked back. And it allowed for *new* things to happen - I cannot imagine King Radical in monochrome, since his design's so complex. And the time portals - in B&W, you'd have just a swirly thing. In color, you've got an otherworldly, contrasting purple.
On one last note, I've quit reading various comics for various reasons. The only ones I've quit because the art sucked, were B&W. Just saying.