I've done a quick run-through of the questions on each tag, and my recommendation is to merge colleagues into coworkers.
Firstly, the two tags have grown since this question was posted. coworkers is now twice the size of colleagues with 22 questions against 12.
More importantly, the meaning of the tags has no obvious difference in the questions asked so far. Every single colleagues question is able to fit into coworkers without losing any meaning. See below:
How could I talk to a coworker about my feeling of being left out by the team?
uses coworker in title
How do I talk to a work friend about their constant no-shows for catch-ups?
talks about people on different team
Dealing with colleagues who think single women as being always available
uses colleague to mean general "people in my office"
Dealing with coworker who is excessively self-deprecating
uses coworker in title
How to stop a colleague from talking to me about work when I'm on my lunch break
contains nothing specific to being in the same departement
How to tell a colleague to take care of how he looks
talks about somebody in a more junior position
How to tell a colleague to not drink water from my bottle
not specific to being the same level/position
Invitation for dinner refused with a strange reason
talking about somebody junior
How do I get my colleagues to turn the radio off?
talking about somebody not directly working with them
Should I reply to the email or just leave it?
talking about their supervisor's colleague, i.e. a coworker of theirs
How to avoid wishing my coworker on her birthday?
uses coworker in the title
How to interact with a colleague if you understand only 5% of what he says?
talking about somebody who works in a different sector
The minor differentiation in meaning that other answers have made so far, is not relevant to the questions that have been asked so far - or the answers they will recieve.
For questions that involve an equal relationship, or one that is imbalanced, these kind of clarifying details are required in the body text already - so the tag does not clear this up.
Worse, many people do not see a distinction between coworker and colleague - so relying on this for having two tags means we will have more incorrectly tagged questions.
Again, as coworkers is technically broader, and all colleagues questions can fit into it without modification - I recommend merging.