I just completed the "Developing form controls" tutorial and found some wrong code:
[C#]public partial class CMSFormControls_ColorSelection : System.Web.UI.UserControl
to
public partial class CMSFormControls_ColorSelection : CMS.FormEngine.FormEngineUserControl
The assembly is actually found in CMS.
FormControls (not CMS.FormEngine) and thus should be:
public partial class CMSFormControls_ColorSelection : CMS.FormControls.FormEngineUserControl