Hi,@Nicholas Piazza. Welcome to Microsoft Q&A.
For custom expression under the color option, just enter the color.
The numbers 1 and 2 in the following picture represent the solid color brush and gradient brush respectively. Currently it is in the solid color brush options and the color is AliceBlue.
SolidColorBrush custom Expression : input lightpink (The background color will change from aliceblue to lightpink)
GradientBrush custom Expression : input color you want((The background color will become the color you enter with the gradient brush)
1.Linear gradient 2.Radial gradient3.reserve gradient stops
For the custom expression under the Offset option, just enter a number of type double.
You could set these options in the Properties window to help you set the color, and you can click the color card to directly select a custom color.
Note: When entering custom expressions in the Visual Studio Property Window, you might need to ensure the expression is syntactically correct and does not break the XAML parser. If you encounter any issues, consider setting the property directly in the XAML file instead of using the Property Window's custom expression feature.
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