property Items.CellBackColor([Item as Variant], [ColIndex as Variant]) as Color

Retrieves or sets the cell's background color.

TypeDescription
Item as Variant A long expression that indicates the item's handle.
ColIndex as Variant A long expression that indicates the column's index, a string expression that indicates the column's caption or the column's key.
Color A color expression that indicates the cell's background color. The last 7 bits in the high significant byte of the color to indicates the identifier of the skin being used. Use the Add method to add new skins to the control. If you need to remove the skin appearance from a part of the control you need to reset the last 7 bits in the high significant byte of the color being applied to the background's part.

To change the background color for the entire item you can use ItemBackColor property. Use the ClearCellBackColor method to clear the cell's background color. Use the BackColor property to specify the control's background color. Use the CellForeColor property to specify the cell's foreground color. Use the ItemForeColor property to specify the item's foreground color.

For instance, the following code shows how to change the left top cell of your group: Group.Items.CellBackColor(Group.Items(0), 0) = vbBlue 

Note: A cell is the intersection of an item with a column. All properties that has an Item and a ColIndex parameters are referring to a cell. The Item parameter represents the handle of an item, and the ColIndex parameter indicates an index ( a numerical value, see Column.Index property ) of a column , the column's caption ( a string value, see Column.Caption property ), or a handle to a cell ( see ItemCell property ). Here's few hints how to use properties with Item and ColIndex parameters:

Group.Items.CellBold(, Group.Items.ItemCell(Group.Items(0), 0)) = True
Group.Items.CellBold(Group.Items(0), 0) = True
Group.Items.CellBold(Group.Items(0), "ColumnName") = True