Change your DataGridView's
DefaultCellStyle
property. It contains settings for selection backcolour.
In the example I did it via subclassing:
class CustomizedDataGridView : DataGridView
{
protected override void OnCreateControl()
{
base.OnCreateControl();
this.DefaultCellStyle.SelectionBackColor = YourDarkerColour;
this.DefaultCellStyle.SelectionForeColor = SomeForecolourStillReadable;
}
}