You need first to navigate to the cell and then set its BackColor property.
Navigation to cell depends on in what place in the code you want to do it.
If you are in the ItemDataBound event, which is the most common place for
handling this sort of tasks, you get a reference to the datagrid item (row)
as a parameter and you can use Cells collection to get to the cell. In
PageLoad or PreRender event you first navigate to the row in the grid and
then to the cell.