PhpStorm light yellow highlight in file explorer / project window, what is it and how can I change the color?

I just started using PhpStorm, and I've noticed when I look into my vendor directory (managed by Composer), I see the background colour is a light yellow and unreadable with my current theme. It like this in all the directories within the vendor folder.

What does it mean? How can I change the background color?

light yellow background in project window composer vendor

Solution

This color usually means that such files are not part of the project itself.

In you case this means that the folder is excluded and then re-added back to the project as External Library (via PHP | Inlude paths functionality) -- this way you do not see ToDos from them in your ToDo list; your own code refactoring does not affect/mess up with libraries and other stuff like that.

You can edit colors at Settings/Preferences | Appearance & Behavior | File Colors.

That particular color belongs to "Non-Project Files" scope.

Few options here.

Change color of that scope entry (by double clicking on it) to any other that would fit your color scheme better.
Disable highlighting files in Project View using background colors by unchecking the Use in Project View option.
Use another GUI Theme that already provides more suitable/right colors.