A simple Trigger
can do this for you in XAML (without any code behind):
<GroupBox Name="groupBox" Grid.Column="0" Grid.Row="1" Margin="2,0,0,0" Header="GroupBox" IsEnabled="False">
<GroupBox.Style>
<Style>
<Style.Triggers>
<Trigger Property="Control.IsEnabled" Value="False">
<Setter Property="Control.Foreground" Value ="#FF6D6D6D" />
</Trigger>
</Style.Triggers>
</Style>
</GroupBox.Style>
<Grid Margin="10,0,0,0">
<Grid.ColumnDefinitions>
<ColumnDefinition />
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height=".250*"/>
<RowDefinition Height=".250*"/>
</Grid.RowDefinitions>
<RadioButton Name="RadioBtn1" VerticalAlignment="Center" ToolTipService.ShowOnDisabled="True" Grid.ColumnSpan="3" Height="14.63">OP1</RadioButton>
<RadioButton Name="RadioBtn2" Grid.Row="1" VerticalAlignment="Center" ToolTipService.ShowOnDisabled="True" Grid.ColumnSpan="3" Height="14.63">OP2</RadioButton>
</Grid>
</GroupBox>