It appears to be H in HSV space for Apple’s standard color.
I thought 210° was just the magic number for iPhone software: the gray-blue color that appears throughout the UI is variations of H=210°. But it also appears that the Apple Mail application uses the same light blue-gray color for the side panel, and for other side panels elsewhere.
As an aside, the blue throughout this blog is at 205°.
As a second aside, for those who don’t know what I’m talking about, here’s the algorithm to convert from HSV to RGB, along with the formulas to go the other way around: Wikipedia: HSL and HSV
Of course it’s important to remember to honor the user’s color selection when building custom controls: NSColor’s +selectedTextBackgroundColor returns the hilite color, and +selectedControlColor returns the color for the face of a selected control, such as the color of a menu item highlight.