My take is that the TOS/TNG uniform color simply reflects which division a person is currently assigned to. It doesn't indicate anything about qualifications or "career track."
The problem with that is that three colors = three departments is way too little. It results in odd combinations such as Security+Engineering+Clerical...
Security, clerical, deck, navigation, weapons, legal, and technical specialties are combined within a single division in the modern US Navy.
In the system currently in use, enlisted personnel are separated into five general categories. Each of the categories is color-coded at the lower rates (pay grades), and is delineated by a particular term:
Airman (Aviation) - Green
Constructionman (Seabees) - Sky Blue
Fireman (Engineering and Hull Repair) - Red
Hospitalman (Medical) - White/Dark Blue (varies by season)
Seaman (Deck/Admin/Navigation/Weapons/Security/Legal) - White/Dark Blue (varies by season)
It's easy to see how this could develop into a Starfleet-like system. Merging of the Airman division into Fireman and Seaman, where appropriate, and consolidating Constructionman with Fireman would yield three divisions which generally correspond with Star Trek's:
Fireman (Engineering, etc.) - Red
Hospitalman (Medical) - White/Dark Blue (varies by season)
Seaman (Deck, etc.) - White/Dark Blue (varies by season)
Using Gold/Khaki, which is associated with officer and chief petty officers in the Navy (and thus with command), to differentiate medical from deck, we would even arrive at the Starfleet color scheme (the Navy has no non-medical scientific ratings, so science could be assumed in the Hospitalman category):
Fireman (Engineering, etc.) - Red
Hospitalman (Medical, Science) - Blue
Seaman (Deck, etc.) - Gold
In TOS terms (and treating officers as being divided the same way):
Kirk - Deck (Command) - Gold
Spock - Science - Blue
Scotty - Engineering - Red
McCoy - Medical - Blue
Sulu - Deck (Helm) - Gold
Uhura - Technical (Communcations) - Gold
Chekov - Navigation - Gold