Styling a combobox's PART_EditableTextBox - wpf

Question

I want to add conditionnal formatting (just font color) to the textbox part of a combobox. According to MSDN, it's the "PART_EditableTextBox" element. A quick search on SO got me started but I now face a problem: it overrides the whole template. According to this SO answer, I can use "BasedOn" to override only specific properties but I've no idea how/where to use it.
This is my current template:
<ControlTemplate x:Key="MyComboBoxTextBox" TargetType="ComboBox" <!--Here?--> >
<TextBox x:Name="PART_EditableTextBox" <!--Maybe Here?-->>
<TextBox.Style>
<Style TargetType="TextBox">
<Style.Triggers>
<Trigger Property="Text" Value="MAL">
<Setter Property="Foreground" Value="DarkOrange"></Setter>
</Trigger>
</Style.Triggers>
</Style>
</TextBox.Style>
</TextBox>
</ControlTemplate>
It works, I can still type in valid values and "MAL" does make the text orange but there's no dropdown anymore.
On MSDN, I found the following:
<TextBox x:Name="PART_EditableTextBox"
Style="{x:Null}"
Template="{StaticResource ComboBoxTextBox}"
HorizontalAlignment="Left"
VerticalAlignment="Bottom"
Margin="3,3,23,3"
Focusable="True"
Background="Transparent"
Visibility="Hidden"
IsReadOnly="{TemplateBinding IsReadOnly}" />
I suppose I should base my template on this "ComboBoxTextBox" but I don't know how to reference it. Do I really need to copy the whole template or is there a way to override a specific property?
EDIT:
On the same MSDN page comboboxTextBox is defined as
<ControlTemplate x:Key="ComboBoxTextBox"
TargetType="{x:Type TextBox}">
<Border x:Name="PART_ContentHost"
Focusable="False"
Background="{TemplateBinding Background}" />
</ControlTemplate>
I don't see how overriding this template removes the dropdown list.

Answer 1

Ok I think I got really confused after reading all of your code and having a really looooooong day at work, I totally missed the point of your question.... which is
I want to add conditionnal formatting (just font color) to the textbox part of a combobox
Well if that's all you want to do, then it's really easy with just a simple style trigger.
I can achieve that with this xaml.
<ComboBox HorizontalAlignment="Center" VerticalAlignment="Center">
<ComboBox.Resources>
<Style TargetType="ComboBox">
<Style.Triggers>
<Trigger Property="Text" Value="MAL">
<Setter Property="Foreground" Value="DarkOrange" />
</Trigger>
</Style.Triggers>
</Style>
</ComboBox.Resources>
<ComboBoxItem>MAL</ComboBoxItem>
<ComboBoxItem>1</ComboBoxItem>
<ComboBoxItem>2</ComboBoxItem>
<ComboBoxItem>3</ComboBoxItem>
</ComboBox>
Hope this helps!