我设法解决它：

Dim dgv As DataGridView = Me.TblCalendarDataGridView

    For i As Integer = 0 To dgv.Rows.Count - 1
        For ColNo As Integer = 4 To 7
            If Not dgv.Rows(i).Cells(ColNo).Value Is DBNull.Value Then

                dgv.Rows(i).Cells(ColNo).Style.BackColor =  vbcolor.blue
            End If
        Next
    Next