So far there were two magic colors to be known by developers of Visual Studio add-ins:
- Almost green (RGB = 0, 254, 0) to get transparency in custom bitmaps of commands. See HOWTO: Create a command with a custom picture using a managed satellite DLL for a XML-based Visual Studio add-in.
- Magenta (RGB = 255, 0, 255) to get transparency in custom bitmaps of toolwindows. See HOWTO: Creating custom pictures for Visual Studio .NET add-ins commands, buttons and toolwindows.
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.