Star Wars: 20 Things You Didn't Know About The Empire Strikes Back
19. Yoda Originally Had Blue Skin
Donald F. Glut's novelisation of The Empire Strikes Back features a number of differences from the final film - such as Darth Vader's lightsaber being blue rather than red - but by far the most peculiar change is the decision to have Yoda's (Frank Oz) skin be blue rather than green.
A passage from the book describes Yoda as such: "[Yoda's] long white hair was parted down the middle and hung down on either side of the blue-skinned head."
But rather than Glut simply going into business for himself, this is actually because Yoda was originally intended to be blue-skinned, before George Lucas changed his mind.
Because novelisations are often written in the earlier stages of production, Glut's book had presumably been completed and possibly even printed before shooting had started.