为什么第二个a还是Red?

//隐式枚举转换示例
namespace EX3._16
{
    enum Color
    {
        Red,Green,Yellow
    }
    class Program
    {
        static void Main(string[] args)
        {
            Color a = Color.Red;
            Console.WriteLine("a equals :{0}", a);
            a = 0;
            Console.WriteLine("a equals :{0}", a);
            Console.Read();
        }
    }
}

enum Color
{
Red,Green,Yellow
}

它的值是
enum Color
{
Red, //=0
Green,//=1
Yellow //=2
}
你2次都给的0.自然是red了