If your grid is a GridDataBoundGrid, then use either the GridBoundColumns (if you explicitly added them) or Binder.InternalColumns to set this value.
//set up the comboboxes
GridStyleInfo style = this.gridDataBoundGrid1.Binder.InternalColumns["masterComboCol"].StyleInfo;
style.DataSource = GetTheMasterTable();
style.DisplayMember = "masterdesc";
style.ValueMember = "id";
style.BackColor = Color.LightBlue;
style.CellType = "ComboBox";
style.HorizontalAlignment = GridHorizontalAlignment.Left;