The monster first appeared onscreen in a film version by Thomas Edison in 1910
www.youtube.com/watch?v=TcLxsOJK9bs
but ever since Boris Karloff portrayed the famous monster in the 1931 film we have this ingrained image of the creature as this big green guy with a flat head. Evidentally he was green because that color looked better on B&W film than yellow did. Yellow? See, when M.S. wrote the book she described the monster as having yellow skin (she also described his features as "beautiful" but how is that supposed to scare movie audiences?)
As for the flat head I heard that Jack Pearce, the make-up artist on the film rationalized that taking out the brain of a cadaver would require taking the top of the skull off. Makes sense. Likewise, putting that brain into another skull would require topping THAT skull too... therefore the recipient of said brain, the monster, would have a topless head -- or a flat topped head. Never mind that presumably to keep dust and bugs out, Dr. Frankenstein would have had the werewithal to REPLACE the removed skull cap, thus returning to his creation that normal "round -headed" look.
If you know of any other explanations, I'm curious to hear them.