' Create a new form.
Dim mdiChildForm As New Form

Private Sub Form1_Load(ByVal sender As Object, _
    ByVal e As System.EventArgs) Handles MyBase.Load

    ' Set the IsMdiContainer property to true.
    IsMdiContainer = True

    ' Set the child form's MdiParent property to 
    ' the current form.
    mdiChildForm.MdiParent = Me

    'Call the method that changes the background color.
    SetBackGroundColorOfMDIForm()
End Sub

Private Sub SetBackGroundColorOfMDIForm()
    Dim ctl As Control

    ' Loop through controls,  
    ' looking for controls of MdiClient type. 
    For Each ctl In Me.Controls
        If TypeOf (ctl) Is MdiClient Then

            ' If the control is the correct type,
            ' change the color.
            ctl.BackColor = System.Drawing.Color.PaleGreen
        End If
    Next

End Sub

    // Create a new form.
    Form mdiChildForm = new Form();

    private void Form1_Load(object sender, System.EventArgs e)
    {

        // Set the IsMdiContainer property to true.
        IsMdiContainer = true;

        // Set the child form's MdiParent property to 
        // the current form.
        mdiChildForm.MdiParent = this;

        // Call the method that changes the background color.
        SetBackGroundColorOfMDIForm();
    }

    private void SetBackGroundColorOfMDIForm()
    {
        foreach ( Control ctl in this.Controls )
        {
            if ((ctl) is MdiClient)

                // If the control is the correct type,
                // change the color.
            {
                ctl.BackColor = System.Drawing.Color.PaleGreen;
            }
        }

    }

To run this example paste the following code in a new form.