The Colors branch under Configuration - Application Layout - File Tabs has the following options:
Tab colors by file specification
If this option is selected the background color of file tabs will be changed to indicate the file type. Users may assign file types to the colors available in the table by double-clicking on a color line and adding an extension (i.e. "*.html"). Additional extensions should be listed immediately following the preceding extension and a semicolon ";" (i.e. "*.php;*.php3;*.html;*.js"). If preferred, users may specify colors for individual files rather than extensions, i.e. "ResolveLine.cpp".