Repro Steps:

1) Create a Project with a MainPage.xaml
2) From the Resources Panel, add any colour of your choosing as a Color Resource within GlobalResources.
3) Introduce a Button to the Stage, right-click on it and navigate to Templates->Create.
4) With the Template open, select the BackgroundBorder, and change its Background colour to the Resource colour created in Step 2.
5) Navigate to the Interactions Panel's 'Setters' tab and under the pre-existing "IsMouseOver=True" Setter, attempt to change the BackgroundBorder's Background colour to a non-dictionary colour value picked using the colour picker.

Observe how it is not possible to select an alternate Solid Color Brush using the ColorPicker.
Observe how the "Clear Local Value" and "Convert to Local Value" functions are non-functional.

Expected Results: Element Properties defined in a Setter can have local values set in the Setter, similar to how it functions in Storyboards.

6) Select a Gradient Brush instead.
Observe how this recolouring technique works as expected.