Baroness began with heavy sludge metal albums, 'Red Album' and 'Blue Record', which had a distinctly yummy heavy psyche/hard rock flavor. This very clearly distinguished their style of sludge from other sludge artists such as Mastodon and Isis.
'Yellow & Green' sees the group all but abandon metal in favor of Captain Beyond-style groovy melodic hard rock with only infrequent blasts of heavy sludge. On occasion, and I must say slightly unsettlingly, it even reminds me of 90s-style alt-metal/post-grunge (albeit clearly more highly skilled).