fix: #4297 use correct opacity for disabled color picker

Author: punker76

src/MahApps.Metro/Themes/ColorPicker/ColorPicker.xaml

@@ -304,7 +304,7 @@
                                 BorderThickness="{TemplateBinding BorderThickness}"
                                 CornerRadius="{TemplateBinding mah:ControlsHelper.CornerRadius}"
                                 IsHitTestVisible="False"
-                                Opacity="0.6"
+                                Opacity="0"
                                 SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"
                                 Visibility="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=(mah:ControlsHelper.DisabledVisualElementVisibility), Mode=OneWay}" />