Cyan: Another magic color for developers of Visual Studio 2012 add-ins
So far there were two magic colors to be known by developers of Visual Studio add-ins:
Ryan Molden (Visual Studio team) posted in a comment here explaining that you can exclude a custom bitmap of a command from being color-inverted in Visual Studio 2012 when using the Dark theme setting the top-right pixel of the picture to the Cyan value (RGB = 0, 255, 255). This allows you to create an add-in that appears colorful both with the Dark and Light themes, provided that the borders appear fine with dark background (you can use ARGB bitmaps with partial opacity if needed).
I have updated the first article to include this information.