gistfile1.txt

Set the outlineView's hightStyle to "none":

outlineView.SelectionHighlightStyle = NSTableViewSelectionHighlightStyle.None;

Then in the NSOutlineViewDelegate:

public override void WillDisplayCell(NSOutlineView outlineView, NSObject cell, NSTableColumn tableColumn, NSObject item)
{
	uint rowNo = (uint)outlineView.RowForItem(item);
	NSColor backgroundColor;
	if (outlineView.SelectedRows.Contains(rowNo)) {
		backgroundColor = NSColor.Yellow;
	}
	else {
		backgroundColor = NSColor.White;
	}
	Console.WriteLine("Cell: {0}", cell);
	var bCell = (NSTextFieldCell)cell;
	bCell.BackgroundColor = backgroundColor;
	bCell.DrawsBackground = true; 
}