One of the first things I noticed in Windows Vista were the toolbars. Certainly they’re prettier now, and the fact that the main menu is on the way out is exciting. But the most interesting thing is that they’re different colors. Windows Explorer gets a blue-green toolbar, Windows Mail and Windows Calendar are blue, Windows Media Player and Windows Photo Gallery are black, and Internet Explorer gets a silver variant. This seems like a cool way to divide up the built-in apps, but the question that comes immediately to my mind is what color a third-party app should choose. It turns out that the Vista User Experience Guidelines are silent on this issue.

Fortunately everettm over at the Shell:Revealed blog was able to answer this question in the comments. To summarize, the recommendation is that all third-party apps use the silver toolbars. The colors are reserved for Windows branding, with blue-green standing for core Windows components like the shell, black for media apps, and blue for PIM/organization apps.
While I appreciate the response, I’m not sure I like that answer. I really like the colored toolbars, and I’d love to have my apps integrate more with the general look and feel of Vista by picking the toolbar color that is most appropriate to my application. For example, if I went to a toolbar-only UI for PNGGauntlet, I might consider going with the black toolbar, since it is an image manipulation app. Hopefully by that time the UX Guidelines will have been updated to give more explicit (and more permissive) guidance on toolbar colors.