When it comes to hair in a professional environment, it seems people either believe it needs to be cropped short or kept in a "librarian" type bun. So I'd like to talk a bit about my personal observations and recommendations...
Be aware of your signal value
First of all, you need to realise there is a signal value in what you wear and what you do to your hair.
You want to look professional while maintaining your unique personality. Beware your brand!
That means you should avoid anything that looks "trendy", "young" or something "statement"-looking.
When in doubt, go by the most basic military rule
If your hair is long enough to reach your collar, keep it up and keep it neat!
Definitely avoid the long, flowing hair everywhere. It sends a signal of someone who is more busy with her hair and might want to avoid stuff like faxes, plotters and copy machines, or other things her hair can get stuck in.
I generally find the military regulations for your hair to be very good to follow: Keep your hair clean, contained and neat. Never do purposely messy styles.This also falls under the next rule:
Avoid the obvious trends
So granted, it's highly unlikely that people in an office know or care that the current trend for hair dye is more of a cool purple than a warm burgundy. But, most people will know and recognize those very popular hair trends like the top knot that was everywhere some time ago and the boxer braids that are everywhere now.
Use the "subtle" trends to play around with, flatter your face with or express your personality.
Avoid the trends that people will actually recognize.
Avoid sharp tools
You probably won't come within arms reach from your coworkers except when you stand over someones shoulder to look at the same monitor. But, avoid objects that are obviously pointy or sharp in your hair. A rounded Ron Quattro is good, a pointy stick isn't. It sends a certain signal, even if it's not a safety issue.
Cycle your styles!
One general truth to everyone who isn't in the longhair circuit is that they can't see the difference between near-identical styles. A Celtic knot looks the same as a Chinese bun, which looks the same as a regular Cinnamon bun, which looks the same as a regular Nautilus bun. People just can't tell.
The same goes for coworkers. Wearing too many vaguely different buns can make people think you're frumpy, unkempt or just boring. You want to look like you care about your signal value, not necessarily about your looks.
What people can recognize though, is things like "Today she is wearing two buns, yesterday it was one" or "Today her bun is braided, yesterday it wasn't".
So this is where you can play around with your hair and express your personality.
Try to cycle your styles according to what is obviously different to the uninitiated.