[arp242]

It's also just an artistic design choice. They tried very hard to make things look "alien" and "otherwordly" as much as possible, ranging from the set design to the costumes to the music to Starfleet sideburns.

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.

[shiroiuma]

Another thing I read somewhere (not sure if it's true) is that TOS used a lot of wacky colors because color TVs were new then, and so they were trying to get more people to watch the show by making it a bit of a showcase, much like "Planet Earth" was popular when FHD TVs were new because it showed the real potential of the technology.