As mentioned by DataMiser, you can change the background color of a cell and the row. If you want to change all the cells of a particular row, you have to loop through all the columns.

Below code may take you in the right direction.
Code:
Private Sub SetBackColor(FlexGrid As MSHFlexGrid, RowNo As Integer, BkColor As ColorConstants)
    Dim ColCount As Integer
    With FlexGrid
        If RowNo >= .Rows Then
            MsgBox "Invalid RowNo"
        End If
        .Row = RowNo
        For ColCount = .FixedCols To .Cols - 1
            .Col = ColCount
            .CellBackColor = BkColor
        Next
    End With
End Sub
Good Luck