Public Overloads ReadOnly Property Item( _
   ByVal row As Integer, _
   ByVal column As Integer _
) As Cell

Dim instance As Cells
Dim row As Integer
Dim column As Integer
Dim value As Cell
 
value = instance.Item(row, column)

public Cell Item( 
   int row,
   int column
) {get;}

The Item property is not available in C#. Use the cell index.

FpSpread1.ActiveSheet.Cells.Item(0, 0).BackColor = Color.Yellow