Hello Kumaran,
Thank you for your post. I have been looking into your question and in order to change the highlighted color brush when the mouse is over some item in the XamMultiColumnComboEditor, you need to change the corresponding brush in the ‘ComboCellControl’ style(CellRowHoverBackgroundBrush). You can find the default styles for the XamMultiColumnComboEditor in :
C:\Program Files (x86)\Infragistics\NetAdvantage 2012.2\WPF\DefaultStyles\XamComboEditor\generic.xaml
I am attaching a sample application(MultiColumnComboEditor.zip) where I have make this brush be red.
Let me know, if you need any further assistance on this matter.