In vb.net you can not change the background color [actually it is called as BackColor property] of MDIParent [IsMdiContainer = true] form, but you can change it by changing the color of MDIChild form that are executing currently to your desired color, so it look like the color of MDIParent is changed.
The Code for it is provided below. Just called this function bgColor() into your load method of your MDIParent.
Code :
#Region "Change Background Color of MDIParent : bgcolor()" 'to change the background color of mdiparent 'for more : https://acomputerengineer.wordpress.com Private Sub bgColor() Dim child As Control For Each child In Me.Controls If TypeOf child Is MdiClient Then child.BackColor = Color.CadetBlue Exit For End If Next child = Nothing End Sub #End Region