For the most part, the tabs are generally black for unchanged, blue for modified and turquoise for switched.
Other colors are the exception.
A lighter or greener shade of blue would be very evident that it is switched plus modified.

The benefit would be to prevent accidental checkins to the head instead of the branch (like happened to me when I opened this issue)

The use-case was - a developer branched several classes in a module that needed modification.
I checked out those classes from the branch and made some modifications, which also required modifications to another class that wasn't in that branch.

I reviewed all of the changes, selected all of the blue tabs, and checked them in.
I didn't realize that one of those was not in the branch but in the head.
So now the head build was broken.
If the color was something other than blue, I would have realized it before hand and prevented this error.

I'm all in favor of features that really help prevent checking in wrong files, but I'm afraid that adding three more subtle shades of file status colors (note that you'll need colors for "switched modified", "switched added" and "switched deleted") will be more confusing than helpful.

There are many ways to break a check in, but those should not be aided by IntelliJ, IntelliJ has always been great about helping me prevent such errors. That's why we pay for IntelliJ when we can get Eclipse for free.

Everyone who is using version control knows very well what the color codes are about.

IntelliJ has always been about "developing with pleasure" and helping developers do their work.
If I have a long checkin and I "forget" to check every file (because this would be the only operation that would require me to make this check), then intelliJ would be helping me break my code.

I have an idea for a compromise - why not provide the "switched updated" color as an option in the Colors and Fonts settings, but set it by default to the same color as the "non-switched" updated.

That way the people who find the change confusing will have it your way, and people like me who consider it a major nuisance would have the ability to change it.

Please? This is really important!

I think that an explicit, modal warning dialog box shown when you try to commit files to different branches at the same time (non-switched and switched, or switched to different branches) will be a much more helpful and less confusing way to avoid this kind of problems. (The dialog should of course have a "Don't show this again" option if such checkins are normal in some people's workflow).

I have another suggestion - instead of color coding, would it be possible to use the same current coding as before, except add an icon in the tab (perhaps a little black circle next to the file name) indicating that it is a changed file.

You could also display that icon in the project view.

This would help for identifying changed files without any new color coding.