Usually you set color for tabs using the tab name and project name regular expressions in the Tab Coloring Rule dialog. If you want to color tabs by type (for example .cs or .xaml) you can use the document paths regex. But then there are form designer [Design] tabs that can’t be matched using the existing rules.
I’ve added the Tab extensions regex rule that complements the Tab name regex rule and lets you assign tab background depending on opened extensions in a tab group:
Tab coloring rule for designer tabs
Designer tabs with a special color
I’ve added the TabExtensions property to the Tab control that consists of the extensions list separated by ‘$’. For example, “Form2 .cs .cs [Design]” tab has “TabName=Form2” and “TabExtensions=.cs$.cs [Design]$”. Note, that a single tab like “Class1.cs” still has the name “Class1” and the extensions “.cs$”.
Download link: Tabs Studio v2.3.5.
Tabs Studio Blog (organizing Visual Studio document tabs) 
Exactly *how* does one use the document regex path to color by extension??
I have tried the following to color code .cs files, nothing works:
.cs
[Cc][Ss]$
\.[Cc][Ss]
\.[Cc][Ss]$
Comment by Milo D. Cooper — May 23, 2011 @ 6:20 pm
Greetings Milo,
The following Tab extensions regexes color code .cs files:
.cs
[Cc][Ss]\$
\.[Cc][Ss]
\.[Cc][Ss]\$
Please, make sure that Tab selected, Previously selected tab and Document checkboxes don’t limit your rule. I recommend you to set them to undefined state for testing.
Comment by Sergey Vlasov — May 23, 2011 @ 7:09 pm