I'll second what meeera said, with a caveat: sometimes I'd rather not see the cover at all. Some publishers create covers that are so generic that they are nothing more than their imprint logo. With some ereaders and ereading software this is worse than useless and ugly: it makes it impossible to identify the book since only the cover is displayed.

When it comes to physical books though, I'll take colour any day. Greyscale would make the home a tad too drab.