The type or namespace name 'X' does not exist in the namespace 'Y' - in VS generated code
So here is my guess.
In your content.ascx
view (or whatever it's named) you have declared:
<%@ Control Language="C#" Inherits="System.Web.Mvc.ViewUserControl<CMSModels.ViewModels.ColourboxViewModel>" %>
instead of
<%@ Control Language="C#" Inherits="System.Web.Mvc.ViewUserControl<CMSModels.ViewModels.ColourBoxViewModel>" %>
I had this error yesterday, and I found these answers, but in my case they didn't help. Instead it turned out that in my client in project properties the Target framework was set to .NET Framework 4 Client Profile. Solution was to change this to full .NET 4. Hope this can help someone, because the error in VS is not very helpfull.