When The Looney Tunes series began in 1930, they were imitations of Walt Disney's "Silly Symphonies." The Merrie Melodies, begun later the same year, copied Max Fleischer's "Bouncing Ball" cartoons.
The Looney Tunes were made in black and white until 1942. In 1942 and 1943, Looney Tunes were produced both in color and in black and white. Four color Merrie Melodies were produced in 1934. After November 1934, all Merrie Melodies were produced in color. So from 1935 to 1942, color, or the lack of it, was the main difference between the two series.
In 1944 and thereafter, all Looney Tunes were produced only in color. The two series were then almost indistinguishable. The only difference was that the rings in the openings of the Looney Tunes were (usually) wider than those in the Merrie Melodies.
Note that in many reissue prints with the Blue Ribbon label, the entries in the Looney Tunes series are falsely identified as Merrie Melodies. The films made before 1948 also have their credits cut in the Blue Ribbon versions. These Blue Ribbon prints have even made it into recent DVD collections, as the original title cards may no longer exist. Several post-1948 cartoons were presented as Blue Ribbon versions, even though their original openings are known to exist.
Leonard Maltin, Of Mice and Magic: A History of American Animated Cartoons, NY, 1987, pp. 224, 419, 427
Dave Mackey's Warner Bros. Cartoons Filmography