Manipulando ViewBag de dentro do Action Filter

gistfile1.cs

using System.Web.Mvc;

namespace Teste.TrocandoDadosComActionFilter.Controllers
{
    public class HomeController : Controller
    {
        [MudaCor]
        public ActionResult Index()
        {
            ViewBag.Cor = "Welcome to ASP.NET MVC!";

            return View();
        }

        public ActionResult About()
        {
            return View();
        }
    }

    public class MudaCorAttribute : ActionFilterAttribute
    {
        public override void OnActionExecuted(ActionExecutedContext filterContext)
        {
            if (filterContext.Controller.ViewBag.Cor != null)
            {
                filterContext.Controller.ViewBag.Cor = "Verde";
            }
            base.OnActionExecuted(filterContext);
        }
    }
}