Fix ColorPickerButton SelectedColor bug (fixes #4367)

Update internal ColorPicker Color when ColorPickerButton SelectedColor changes

Author: Marv51

Microsoft.Toolkit.Uwp.UI.Controls.Input/ColorPicker/ColorPickerButton.cs

@@ -81,7 +81,7 @@ public Color SelectedColor
         /// Identifies the <see cref="SelectedColor"/> dependency property.
         /// </summary>
         public static readonly DependencyProperty SelectedColorProperty =
-            DependencyProperty.Register(nameof(SelectedColor), typeof(Color), typeof(ColorPickerButton), new PropertyMetadata(null));
+            DependencyProperty.Register(nameof(SelectedColor), typeof(Color), typeof(ColorPickerButton), new PropertyMetadata(null, new PropertyChangedCallback(SelectedColorChanged)));
 
 #pragma warning disable SA1306 // Field names should begin with lower-case letter
         //// Template Parts
@@ -142,6 +142,14 @@ protected override void OnApplyTemplate()
             }
         }
 
+        private static void SelectedColorChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
+        {
+            if (d is ColorPickerButton instance && !(instance.ColorPicker is null))
+            {
+                instance.ColorPicker.Color = instance.SelectedColor;
+            }
+        }
+
         private void ColorPicker_ColorChanged(Microsoft.UI.Xaml.Controls.ColorPicker sender, Microsoft.UI.Xaml.Controls.ColorChangedEventArgs args)
         {
             SelectedColor = args.NewColor;