You can handle PrepareViewStyleInfo to do this.
private void gridControl1_PrepareViewStyleInfo(object sender, GridPrepareViewStyleInfoEventArgs e)
{
if(e.ColIndex > 0 && e.RowIndex > 0)
{
if(this.gridControl1.ReadOnly || e.Style.ReadOnly)
e.Style.BackColor = Color.FromArgb(240, 240, 240);
}
}