Failed to Create Component .. Type is not Marked as Serializable

You can use this :

[Browsable(false)]
[EditorBrowsable(EditorBrowsableState.Never)]
[DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
public List<ColorData> PaletteColors { get; set; }

But you also can add this, if your form is localised :

 [System.ComponentModel.Localizable(false)]

You can try this if you want to be browsable in design mode

[System.Xml.Serialization.XmlArray]
    public List<Pen> PenList
    {
        get { return penList; }
        set { penList = value; }
    }

No doubt it is some extra attributes are not serializable by designer to show it on the designer surface.

Try adding these attributes to non-serializable properties of the user control:

[Browsable(false)]
[EditorBrowsable(EditorBrowsableState.Never)]
[DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
public List<ColorData> PaletteColors { get; set; }