Is viewer the code behind for your user control/window? If it's not, then you can't set a property on a control like this because it's not actually part of that view.
[Edit]Amended to show the call to InitializeComponent()
Imports System.Windows.Media
Imports System.Windows.Media.Media3D
Imports HelixToolkit.Wpf
Imports System.Windows.Media.Drawing
Public Class viewer
Public Sub New()
InitializeComponent()
End Sub
Public Property BackColor() As Color
Get
Return Canvas1.Background
End Get
Set
Canvas1.removed = value
End Set
End Property
End Class