property Items.ItemBackColor(Item as HITEM) as Color

Retrieves or sets a background color for a specific item.

TypeDescription
Item as HITEM A long expression that indicates the handle of the item.
Color A color expression that indicates the item's background color.

To change the background color for a cell you can call CellBackColor. To change the background color of the entire control you can call BackColor property of the control.  

The following sample changes the background color for cells in the first column:

Private Sub ExplorerTree1_AddItem(ByVal Group As EXPLORERTREELibCtl.IGroup, ByVal Item As EXPLORERTREELibCtl.HITEM)
    With Group.Items
        .CellBackColor(Item, 0) = vbBlue
    End With
End Sub