vb.net - How To Change Background Color of RichTextBox After Assigning into DataGridView

I would like to change the background color of RichTextBox which is in a cell of DataGridView.

I have tried use

Me.dgvPartTracking.Item(columnIndex, rowIndex).Style.BackColor = Color.LightGreen

Me.dgvPartTracking.Item(columnIndex, rowIndex).Style.ForeColor = Color.Black

but the result is only the background of cell is changed, and the RichTextBox beackground color is still remaining in white color

The output is as below:
Sample Result

The Method I used to assign the RichTextBox into DataGridView.

*I use looping to add the columns and rows as below

        Dim Col As New DataGridViewRichTextBoxColumn
        Col.Name = "schedule" & columnCount
        Col.HeaderText = "" & columnCount
        Col.DefaultCellStyle.WrapMode = Windows.Forms.DataGridViewTriState.True
        Col.DefaultCellStyle.Alignment = DataGridViewContentAlignment.TopLeft
        Col.Width = 195
        Col.ReadOnly = True
        Col.SortMode = Windows.Forms.DataGridViewColumnSortMode.NotSortable

        Col.Resizable = Windows.Forms.DataGridViewTriState.True
        Col.AutoSizeMode = Windows.Forms.DataGridViewAutoSizeColumnMode.NotSet
        Col.Visible = True
        Me.dgvPartTracking.Columns.Add(Col)
        Me.dgvPartTracking.Rows.Add(1)

I didn't set the background color in this code yet, because I would like to change the different background colors for each cells in DataGridView afterward

Answer

I have just solved this problem by using the way as below:

Dim cell As New DataGridViewRichTextBoxCell
cell.setBackColor(Color.LightGreen)
Me.dgvPartTracking.Item(columnIndex, rowIndex) = cell
Me.dgvPartTracking.Item(columnIndex, rowIndex).Style.BackColor = Color.LightGreen
Me.dgvPartTracking.Item(columnIndex, rowIndex).Style.ForeColor = Color.Black

Thank You for answering to me @jmcilhinney, I got the idea from your comment