public ActionResult Index()
{
      IEnumerable<Colour> colours = null;

      if (isUserAdmin())
      {
          colours = db.Colours;
      }
      else
      {
          string userId = User.Identity.GetUserId();
          IEnumerable<string> userRoleIds = db.Users.Find(userId).Roles.Select(x => x.Id);
          colours = db.ColourRoles.Where(x => userRoleIds.Contains(x.RoleId)).Select(y => y.Colour).Distinct();
      }

      ViewData["Colours"] = colours;
      return View();
}

[TestClass]
public class ControllerUnitTest {

    [TestMethod]
    public void ControllerIndexTest() {
        // Arrange
        var username = "[Test_Username_Here]";
        string[] roles = new[] { "FakeRoleId1", "FakeRoleId2" };
        var controller = new CollorsController(new FakeDbContext());
        controller.ControllerContext = new ControllerContext() {
            Controller = controller,
            RequestContext = new RequestContext(new MockHttpContext(username, roles), new RouteData())
        };

        // Act
        var actionResult = controller.Index();
        var viewResult = actionResult as ViewResult;

        // Assert
        Assert.IsNotNull(viewResult);
        Assert.IsTrue(viewResult.ViewData.ContainsKey("Colours"));
    }

    private class MockHttpContext : HttpContextBase {
        private readonly IPrincipal user;

        public MockHttpContext(string username, string[] roles = null) {
           var identity = new GenericIdentity(username, string.Empty);
           var principal = new GenericPrincipal(identity, roles ?? new string[] { });
           user = principal;
        }

        public override IPrincipal User {
            get {
                return user;
            }
            set {
                base.User = value;
            }
        }
    }
}

var username = "[Test_Username_Here]";
string[] roles = new[] { "FakeRoleId1", "FakeRoleId2" };
var fake = Isolate.Fake.AllInstances<HttpContextBase>();
var user = new ClaimsPrincipal(new GenericPrincipal(username, roles ?? new string[] { }));
Isolate.WhenCalled(() => fake.User).WillReturn(user);