change color of ellipse

You need a SolidColorBrush[^] if you want to change the fill color of your ellipse. Set the necessary property of your SolidColorBrush object and then assign it to the Fill property of your ellipse. Of course you need to set some condition on which color to set on your SolidColorBrush object when a value is selected on your combobox. This[^] could probably help.

Basic idea is: populate your combo box with all brushes you need. For list item content, use members of System.Windows.Media.Brushes, see http://msdn.microsoft.com/en-us/library/system.windows.controls.combobox.aspx[^]. In the handler of the event of ComboBox.SelectionChanged, type-cast the content of the combo box element to SolidColorBrush and assign it to the property Fill of the Ellipse.

For a combo box item, you can use as complex content as you want; it is only important that you have object of the type SolidColorBrush or Brush at some level of nesting. In this way, you can show color and, for example, its name in the combo box list item.

Here is a very simple example:

<ComboBox>
   <ComboBoxItem>
      <TextBlock Background="Red">Red</TextBlock>  
   </ComboBoxItem>
   <ComboBoxItem>
      <TextBlock Background="Yellow">Yellow</TextBlock>
   </ComboBoxItem>
   <ComboBoxItem>
      <TextBlock Background="AliceBlue">Alice Blue</TextBlock>
   </ComboBoxItem>
</ComboBox>

It's not perfect, just gives you the idea what can you have for a control (combo box) used for color selection.

All trick is in the design of a ComboBoxItem content and using it. So, read this:
http://msdn.microsoft.com/en-us/library/bb613548.aspx[^].

Another different level of WPF "custom drawing" is styling and templating. Read http://msdn.microsoft.com/en-us/library/ms745683.aspx[^].

—SA