public class Color
{
     private static string[] _colors = {"Red", "Green", "Blue"};
     public static int Index { get; set; }

     public static string GetNextColor()
     {
         var retVal = _colors[Index];

         Index++;

         return retVal;
     }
}

[HttpGet]
 public string TestColor()
 {
     Color.Index = 0;
     return string.Format("First color: {0} Second color: {1} Third color: {2}", Color.GetNextColor(), Color.GetNextColor(), Color.GetNextColor());
 }

<string xmlns="http://schemas.microsoft.com/2003/10/Serialization/">
First color: Red Second color: Green Third color: Blue
</string>