Since the release of Visual Studio 2015 Update 3, the VS XAML Designer pop up brush editor quit working in the Properties window when properties were sorted by name, or when properties of type brush were not in the Brush category. This issue affected both Visual Studio and Blend for Visual Studio 2015.
Microsoft posted the fix in this cumulative servicing update:
Microsoft Visual Studio 2015 Update 3 (KB3165756)
I have installed the update and tested various controls and use cases and can confirm the fix corrected the problem.
Have a great day.
Just a grain of sand on the worlds beaches.