Right-click the control in Design view
Choose Edit Style> Edit Copy.
A style will be created for the control in xaml inside Page.Resources as ComboBoxStyle1 (the name depends on your x: name)
You will find
<TextBlock x:Name="DropDownGlyph" Grid.Column="1" Foreground="{StaticResource ComboBoxArrowForegroundThemeBrush}" FontWeight="Bold" FontSize="{StaticResource ComboBoxArrowThemeFontSize}" FontFamily="{StaticResource SymbolThemeFontFamily}" HorizontalAlignment="Right" IsHitTestVisible="False" Margin="0,0,6,4" Text="" VerticalAlignment="Center"/>
Change the foreground to the color you want. for example: Foreground = "Red" or any other resource binding.
App.xaml, - ,
<ComboBox HorizontalAlignment="Left" Margin="187,130,0,0" VerticalAlignment="Top" Width="120" Style="{StaticResource ComboBoxStyle1}"/>