Question

I do have a control template for WPF button. Within the template I can use the Fillcolor and Backcolor of the button. But is it possible to define a third color that can be used in the template and later on in a real button?

Here an example of a circular button. I would like to add a color for the ToggleButton.IsChecked state.

<ControlTemplate x:Key="MyButtonTemplate" TargetType="{x:Type ToggleButton}">
    <Grid>
        <Ellipse x:Name="outerCircle" Width="Auto" Height="Auto" StrokeThickness="4" Fill="White" Stroke="Gray"/>
        <Label x:Name="content" Content="{TemplateBinding Content}" HorizontalAlignment="Center" VerticalAlignment="Center" Foreground="Gray"/>
    </Grid>
    <ControlTemplate.Triggers>
        <Trigger Property="IsChecked" Value="True">
            <Setter TargetName="outerCircle" Property="Fill" Value="{Binding Path=Foreground, RelativeSource='{RelativeSource TemplatedParent}'}"/>
        </Trigger>
        <Trigger Property="IsEnabled" Value="False">
            <Setter TargetName="outerCircle" Property="Fill" Value="LightGray"/>
        </Trigger>
        <Trigger Property="IsMouseOver" Value="True">
            <Setter TargetName="outerCircle" Property="Fill" Value="{Binding Path=Foreground, RelativeSource='{RelativeSource TemplatedParent}'}"/>
            <Setter TargetName="content" Property="Foreground" Value="Gray"/>
        </Trigger>
    </ControlTemplate.Triggers>
</ControlTemplate>

Answer 1

I am not sure wether or not you want to bind this to a property or just use a brush resource, Here is an example using a Resource:

<Window x:Class="WpfApplication1.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="MainWindow" Height="350" Width="525">
    <Window.Resources>
        <SolidColorBrush x:Key="ControlPressedColor">#FF211AA9</SolidColorBrush >
        <ControlTemplate x:Key="MyButtonTemplate" TargetType="{x:Type ToggleButton}">
            <Grid>
                <Ellipse x:Name="outerCircle" Width="Auto" Height="Auto" StrokeThickness="4" Fill="White" Stroke="Gray"/>
                <Label x:Name="content" Content="{TemplateBinding Content}" HorizontalAlignment="Center" VerticalAlignment="Center" Foreground="Gray"/>
            </Grid>
            <ControlTemplate.Triggers>
                <Trigger Property="IsChecked" Value="True">
                    <Setter TargetName="outerCircle" Property="Fill" Value="{StaticResource ControlPressedColor}"/>
                </Trigger>
                <Trigger Property="IsEnabled" Value="False">
                    <Setter TargetName="outerCircle" Property="Fill" Value="LightGray"/>
                </Trigger>
                <Trigger Property="IsMouseOver" Value="True">
                    <Setter TargetName="outerCircle" Property="Fill" Value="{Binding Path=Foreground, RelativeSource='{RelativeSource TemplatedParent}'}"/>
                    <Setter TargetName="content" Property="Foreground" Value="Gray"/>
                </Trigger>
            </ControlTemplate.Triggers>
        </ControlTemplate>
    </Window.Resources>
    <Grid>
        <ToggleButton Template="{StaticResource MyButtonTemplate}"  Height="50" Width="50"></ToggleButton>
    </Grid>
</Window>

---

Base on the clarification that you wanted to be able to set the property on the ToggleButton you will need to use a Dependecy Property here is a quick example:

Custom Control

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace WpfApplication1
{

    public class CustomControl2 : System.Windows.Controls.Primitives.ToggleButton 
    {
        public static readonly DependencyProperty myFillColorProperty = DependencyProperty.Register("myFillColor",typeof(SolidColorBrush),typeof(System.Windows.Controls.Primitives.ToggleButton));    
        static CustomControl2()
        {
            DefaultStyleKeyProperty.OverrideMetadata(typeof(CustomControl2), new FrameworkPropertyMetadata(typeof(CustomControl2)));
        }

        public SolidColorBrush myFillColor
        {
            get { return (SolidColorBrush)GetValue(myFillColorProperty); }
            set { SetValue(myFillColorProperty, value); }

        }
    }
}

MainWindow.xaml

<Window x:Class="WpfApplication1.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:my ="clr-namespace:WpfApplication1"
    Title="MainWindow" Height="350" Width="525">
    <Window.Resources>
        <ControlTemplate x:Key="MyButtonTemplate" TargetType="{x:Type ToggleButton}">
            <Grid>
                <Ellipse x:Name="outerCircle" Width="Auto" Height="Auto" StrokeThickness="4" Fill="White" Stroke="Gray"/>
                <Label x:Name="content" Content="{TemplateBinding Content}" HorizontalAlignment="Center" VerticalAlignment="Center" Foreground="Gray"/>
            </Grid>
            <ControlTemplate.Triggers>
                <Trigger Property="IsChecked" Value="True">
                    <Setter TargetName="outerCircle" Property="Fill" Value="{Binding Path=myFillColor, RelativeSource='{RelativeSource TemplatedParent}'}"/>
                </Trigger>
                <Trigger Property="IsEnabled" Value="False">
                    <Setter TargetName="outerCircle" Property="Fill" Value="LightGray"/>
                </Trigger>
                <Trigger Property="IsMouseOver" Value="True">
                    <Setter TargetName="outerCircle" Property="Fill" Value="{Binding Path=Foreground, RelativeSource='{RelativeSource TemplatedParent}'}"/>
                    <Setter TargetName="content" Property="Foreground" Value="Gray"/>
                </Trigger>
            </ControlTemplate.Triggers>
        </ControlTemplate>
    </Window.Resources>
    <Grid>
        <my:CustomControl2 myFillColor="Red" Template="{StaticResource MyButtonTemplate}"  Height="50" Width="50"></my:CustomControl2>
    </Grid>
</Window>