For example sky can be any colour except blue, and doors can open every which way (except downwards, because that would require digging a hole in the set) and they make all sorts of sounds – as long as they're not "just" doors (they're "alien" doors!) They used those awkward holsters for phasers because pockets looked too "common".

Other more technical factors probably played a factor as well, but this also worked two ways and limited creativity: IIRC the origin skin colour for Vulcans was supposed to be red, but was changed to the off-white because the red didn't work on B&W.