Using specialization to test the existence of a value within a class

Lets say I have the following class:

struct Color
{
   enum {
      blue,
      red,
      unknown
   };
};

I would like to test in compile time if the class Color has the unknown value.

I thought the following could work but it is currently returning an incorrect result.

#include <iostream>

template <typename T, typename U = T>
struct test
{
  static constexpr bool value = false;
};

template <typename T>
struct test<T, decltype(T::unknown)>
{
  static constexpr bool value = true;
};

int main()
{
  std::cout << test<Color>::value << std::endl; //prints 0
}

Could you provide some insight on how I can accomplish the check? Or what is wrong with my example?

Thanks