I just read Shelly's Frankenstein. I was surprised at how well-spoken and erudite the monster was in the book and not the mumbling stumbling green thing we saw in the movies.
When did the green version become the one that everyone knows?
In the novel, the monster is described as having yellowish skin, and in Universal Pictures 1931 adaptation, the promotional material (posters etc) confirms to this, however the make-up artist Jack Pierce created a look that included other key features of the 'trope Frankenstein', including the square head and bolts through the neck.
He also gave the actor green make-up, which would show up as a ghostly white on the panchrome black and white film of the day. This skin colour, as well as the other features, were repeated in a number of other Universal monster films that became culturally embedded.
The technical limitations of black and white film led to many set, costume and make-up decisions being made with the grey value of objects in mind, rather than their true hue, as earlier orthochrome film showed red-orange colours as dark grey almost to black, while later panchrome film, while better in many ways, had similar issues with red-blue.
Others have commented on the film versions in detail but Frankenstein is an interesting case in the adaptations being far, far, better known than the source text. There's a detailed breakdown of the novel's publication history in William St.Clair's The Reading Nation in the Romantic Period. To give a short breakdown of it here:
There are two main versions of Shelley's novel, the original published in 1818 and the revised version from 1831. The original is a spiky, consciously philosophical novel (a "novel of ideas" in the language of the time) that makes a case for radicalism tempered by responsibility. The 1831 text is a bit more conservative, the supporting cast become businessmen and engineers rather than poets. The 1818 text was published in a small run of 500 copies - not unusual for a first novel - which sold out in a respectable amount of time. It was then "discovered" by the playwright Richard Brinsley Peake who wrote his own version for the stage, without acknowledgement (copyright was easy to avoid across mediums) under the title Presumption, or, The Fate of Frankenstein which pushes the now familiar "don't mess with nature" theme. I think Peake's monster was blue, but don't quote me on that. Presumption was a huge hit, and many more people saw the play than ever read the 1818 book - though Shelley was in Italy at the time, her father William Godwin persuaded a publisher to print a new edition of the 1818 to capitalise on the success. Presumption spawned imitators and parodies but Shelley saw very little return on her original work.
Jump forward to 1830 and a new publisher - Richard Bentley - is about to revolutionise fiction publishing by issuing new, affordable, editions of classic works printed using stereotyping (casting whole page plates instead of assembling one page at a time from common letter/word stock - much faster but with high overheads). He asks Godwin for the rights to his Caleb Williams (he gets them) and Godwin and Shelley convince Bentley to publish a new - revised Frankenstein too. Revised editions were a common thing in publishing at the time, essentially a bit of marketing to convince people to buy the same book again (consider Special Edition DVDs today) but Shelley pushed the boat out by refashioning the novel's themes. Why she did this is still an open question: had her politics changed, or did she just think something closer to Peake's interpretation would sell better? It did sell though, and the 1831 text became the standard until relatively recently (now you can buy both).
James Whale's iconic 1931 film is, perhaps unsurprisingly, based on yet another stage version by Peggy Webling (1927) , which in turn had been adapted for the US stage by John L. Balderston. You can see how many layers of interpretation and reinterpretation there are between any pop culture version of the monster and Mary Shelley's book.