comics printed in magazine format (larger & generally on higher quality paper, targetting a non-kid audience & bypaassing the comics code) such as mad magazine & heavy metal magazine were also in b&w.
Not entirely accurate. The early MAD started out as a US standard-format color comic, then switched to a b&w magazine format (thus escaping the scrutiny of the Comics Code Authority) for most of its publishing history, but after officially becoming a DC Comics publication, it switched to color (and later started carrying -- BLECCH! -- advertising). And of course, as with the color comic book format MAD, the b&w magazine also inspired a hoard of imitators, most notably SICK (edited by Joe Simon) and CRACKED (the longest-running of all the MAD wannabees).
HEAVY METAL (and its Marvel counterpart, EPIC) were in full color on quality paper from the very start. I suspect you're confusing those with the Warren Publications comic magazines (CREEPY, EERIE, VAMPIRELLA, THE SPIRIT, 1984, THE ROOK, GOBLIN, and others) and/or Marvel's attempt in the 1970s to take a bite out of Warren's market with such titles as SAVAGE SWORD OF CONAN, DRACULA LIVES, MONSTERS UNLEASHED, DEADLY HANDS OF KUNG-FU, and UNKNOWN WORLDS OF SCIENCE FICTION (among others), which
were in b&w, and on very cheap newsprint (but perhaps a little heavier paper than those later 1970s/1980s color comic books). But even the Warren magazines were known to carry color insert sections on occasion. HEAVY METAL continues (still in full color magazine format) even to this day, although with sales greatly diminished from its glory years, the early 1980s, and now under the editorial aegis of Grant Morrison.
As for Marvel, moving from the 1970s into the 1980s, most of their b&w magazine line had been cancelled, with the notable exception of the seemingly-unkillable SAVAGE SWORD OF CONAN. The NBC TV series THE INCREDIBLE HULK starring Bill Bixby and Lou Ferrigno inspired Marvel to launch the b&w RAMPAGING HULK magazine, but with issue #10, this was converted to a color magazine with the shortened title THE HULK! -- in which format it lasted to issue #27. After EPIC magazine, Marvel's b&w line was diminished to just SAVAGE SWORD OF CONAN, and the occasional short-lived experimental title. MARVEL PREVIEW, a rotating showcase-style b&w title which had begun in the 1970s, was changed to BIZARRE ADVENTURES in 1981 with its 25th issue, but with the (Feb. 1983-dated) 34th issue (and the first to be in full color), the title was cancelled. Marvel seemingly had better success in the 1980s with a line of color one-shot movie adaptations under the sequential banner of MARVEL SUPER SPECIAL, and the MARVEL GRAPHIC NOVEL series which were essentially squarebound, cardboard-covered magazines with no advertising or editorial matter, featuring a single long story of 50 pages or so (but still sequentially numbered and maintaining a regular publication schedule). Oddly or not, Marvel chose not to use the Graphic Novel line to showcase its most popular characters or titles (with some few exceptions), instead concentrating on a mixture of 2nd-tier characters and entirely original one-shots. It seems like they were probing to see which characters in their catalog that had met with relatively lesser success as a standard-format color comic might not prove to be more successful with an older audience that had more spending money.