Star Trek

Many of the key section chiefs (for want of a better word) had a station on the bridge. Scotty did in TOS (but McCoy didn't - although it never stopped him hanging around!). Spock was always there of course but I guess it would have been difficult to show a separate sciences areas on the ship. He was needed on the bridge. Ditto Uhura. The colour thing breaks down anyway since although yellow was the command colour (Kirk/Sulu/Chekov) - second in command of the ship was Mr. Spock (blue/sciences) and third in command was Mr. Scott (red).

Yes. He was the ship's Operations Officer, which included supervision of the helm and sensors responsibilities and co-ordinating the ships resources to other departments.

In Data's case (and the same with Harry Kim but not Miles O'Brien) he also doubled up as the ship's Science Officer. Data was also the second officer, but his primary position was as Operations Officer therefore the correct uniform for him was gold.

Yellow

I read some where that Data's uniform color was supposed to be blue but due to spiner's make up clashing with the blue it was changed to gold.

I personally think that Data's uniform should be yellow because his first job was the opertional officer

even though he wore GOLD(not yellow), it was nice seeing him in red in the alternate universes episodes!

it was always a nice change to see any character in  a different color

picard in BLUE "TAPESTRY"

Riker in GOLD "SECOND CHANCES"

Data is the ops guy and engineer. he should be yellow.

I don't think the Enterprise had an executive/first officer in TOS like we had in TNG. Spock didn't seem to have the command responsibilities that Riker had, for example. Recall that when Data became First Officer in TNG for a few seconds that he changed his uniform to red. This wasn't required for the rank of commander since Crusher and Troi both had that rank, so clearly the ship's first officer had some important command role that wasn't around in the TOS era.

Is this about rank or fashion? 

Anywho, if it's about rank, then of course. He's Commander, not Captain. 

Fashion, I still have the same answer.

Actually, I think Spock and Data are as different as Kirk and Janeway. Totally opposite in my opinion. XD 

command is yellow...blue is science/medical...red is your dead.

I had been wondering this myself.  A quick, related question: Do the Star Trek "yellow" or "gold" uniforms look green to anyone else?  Everything described as "gold" on TNG looks greenish to me--yellow uniforms, Data's eyes, etc.  My other half tells me that it's typical of females to see some shades of gold as green.  Do I exhibit this form of color-blindness, and does anyone else?

I read this also.  He was originally intended to be the science officer, but the blue clashed with the makeup so they switched him to operations.  Most of what he did was science, with engineering as a secondary skill.  This also had the problem of this ship of exploration not having an identified chief science officer.  Spock would still have been in blue in the TNG timeframe.  Whenever they used the color code uniforms, blue was science & medical.  The change between TOS & TNG was that red & yellow got swapped.  I read somewhere that the reason for that is concern over the decades long running joke about "red shirts" when TNG was being developed.  As for red being for "command ranks".  There are officers & crew that wear red, but are not of a "command rank", but are simply on a "command path", Ensign Ro, for example.

I read (somewhere) that Data was originally just meant to be the "second officer" and would have the third center seat next to Picard. Then they decided to give that chair to Troi, and put Data in one of the consoles in front. That meant coming up with a job for him, so they invented the "operations officer", which no one really understands.

Putting him in a yellow uniform was probably an asthetic choice: as others have said, the darker red and blue colors would have clashed with his gold make-up. I think that was the reason behind the color changes in TNG anyway: the producers probably tested various costumes on the actors and decided that red looked best on Stewart and Frakes, and so on. I've often wondered about this.

Fascinating. I have never heard of this before!