最も可能性の高い理由は、ColorPicker コントロールがそのテンプレート内に何らかの要素を期待していることです。通常、そのような要素には「PART_...」で始まる特別な名前があります。どの要素が見逃されているかについてより多くの情報を得るには、デフォルトのカラーピッカーテンプレートを調べて、そこにある要素とその名前を見つける必要があると思います.
考えられる修正シナリオ: ColorPicker コントロールに必須のコントロールの名前を見つけて、この名前を四角形に付けます。
ColorPicker のデフォルト テンプレートは次のとおりです。
<ControlTemplate TargetType="{x:Type local:ColorPicker}">
<Grid>
<ToggleButton x:Name="PART_ColorPickerToggleButton"
IsTabStop="True"
MinHeight="22"
Background="{TemplateBinding Background}"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}"
VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"
HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
Padding="{TemplateBinding Padding}"
IsChecked="{Binding IsOpen, RelativeSource={RelativeSource TemplatedParent}, Mode=TwoWay}"
IsHitTestVisible="{Binding IsOpen, RelativeSource={RelativeSource TemplatedParent}, Converter={StaticResource InverseBoolConverter}}"
Style="{TemplateBinding ButtonStyle}">
<Grid Margin="2">
<Border x:Name="ColorOnly" Style="{StaticResource ColorDisplayStyle}" />
<Border x:Name="ColorAndName" Background="White" Visibility="Hidden">
<StackPanel Orientation="Horizontal">
<Border HorizontalAlignment="Left" Width="20" Margin="2,1,4,1" Style="{StaticResource ColorDisplayStyle}" BorderThickness="1" BorderBrush="#FFC9CACA" />
<TextBlock Text="{Binding SelectedColorText, RelativeSource={RelativeSource TemplatedParent}}" VerticalAlignment="Center" />
</StackPanel>
</Border>
</Grid>
</ToggleButton>
<Popup x:Name="PART_ColorPickerPalettePopup" VerticalAlignment="Bottom" IsOpen="{Binding ElementName=PART_ColorPickerToggleButton, Path=IsChecked}" StaysOpen="False" AllowsTransparency="True" Focusable="False" HorizontalOffset="1" VerticalOffset="1" PopupAnimation="Slide">
<Border BorderThickness="1" Background="{StaticResource PopupBackgroundBrush}" BorderBrush="{StaticResource ColorPickerDarkBorderBrush}" Padding="3">
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="Auto" />
<RowDefinition />
<RowDefinition Height="Auto" />
</Grid.RowDefinitions>
<Grid x:Name="_gridStandardColorsHost" Margin="4">
<Grid.RowDefinitions>
<RowDefinition Height="Auto" />
<RowDefinition Height="Auto" />
<RowDefinition Height="Auto" />
<RowDefinition Height="Auto" />
</Grid.RowDefinitions>
<!-- Available Colors -->
<Grid Grid.Row="1" Visibility="{TemplateBinding ShowAvailableColors, Converter={StaticResource BooleanToVisibilityConverter}}">
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="Auto" />
<RowDefinition />
</Grid.RowDefinitions>
<TextBlock Text="{TemplateBinding AvailableColorsHeader}" Background="AliceBlue" Padding="2" Margin="0,0,0,1" />
<ListBox x:Name="PART_AvailableColors"
Grid.Row="1"
ItemsSource="{Binding AvailableColors, RelativeSource={RelativeSource TemplatedParent}}"
Style="{StaticResource ColorListStyle}" />
</Grid>
</Grid>
<!-- Standard Colors-->
<Grid Grid.Row="2" Visibility="{TemplateBinding ShowStandardColors, Converter={StaticResource BooleanToVisibilityConverter}}">
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="Auto" />
<RowDefinition Height="Auto" />
</Grid.RowDefinitions>
<TextBlock Text="{TemplateBinding StandardColorsHeader}" Background="AliceBlue" Padding="2" Margin="0,1,0,1" />
<ListBox x:Name="PART_StandardColors"
Grid.Row="1"
ItemsSource="{Binding StandardColors, RelativeSource={RelativeSource TemplatedParent}}"
Style="{StaticResource ColorListStyle}" />
</Grid>
</Grid>
<!-- Recent Colors-->
<Grid Grid.Row="3" Margin="0,1,0,1" Visibility="{TemplateBinding ShowRecentColors, Converter={StaticResource BooleanToVisibilityConverter}}">
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="Auto" />
<RowDefinition Height="Auto" />
</Grid.RowDefinitions>
<TextBlock Text="{TemplateBinding RecentColorsHeader}" Background="AliceBlue" Padding="2" Margin="0,1,0,1" />
<ListBox x:Name="PART_RecentColors"
Grid.Row="1"
ItemsSource="{Binding RecentColors, RelativeSource={RelativeSource TemplatedParent}}"
Style="{StaticResource ColorListStyle}" />
</Grid>
</Grid>
</Grid>
<!-- ColorCanvas -->
<Grid x:Name="_colorCanvasHost" Visibility="Collapsed">
<local:ColorCanvas x:Name="PART_ColorCanvas"
Background="Transparent"
BorderThickness="0"
SelectedColor="{Binding SelectedColor, RelativeSource={RelativeSource TemplatedParent}}" />
</Grid>
<Separator Grid.Row="1" HorizontalAlignment="Stretch" Margin="5,0,5,0" />
<!-- More Colors Button -->
<ToggleButton x:Name="_colorMode" Grid.Row="2" Content="Advanced" Margin="5" Visibility="{TemplateBinding ShowAdvancedButton, Converter={StaticResource BooleanToVisibilityConverter}}" />
</Grid>
</Border>
</Popup>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="DisplayColorAndName" Value="True">
<Setter TargetName="ColorOnly" Property="Visibility" Value="Collapsed" />
<Setter TargetName="ColorAndName" Property="Visibility" Value="Visible" />
</Trigger>
<Trigger SourceName="_colorMode" Property="IsChecked" Value="True">
<Setter TargetName="_colorMode" Property="Content" Value="Standard" />
<Setter TargetName="_colorCanvasHost" Property="Visibility" Value="Visible" />
<Setter TargetName="_gridStandardColorsHost" Property="Visibility" Value="Collapsed" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>