how about

switch(c1) {
 case "blue":
 case "green":
 case "red":
   return true;
 default:
   return false;
}

More generally, you could use
return re.IsMatch(color1);

If it was an enum, then the Enum helper class can do a lot (i.e. list
available items, parse items, etc); e.g.

       enum MyColors { Blue, Red, Green };
       static void Main(string[] args) {
           Console.WriteLine(IsColor("blue"));
           Console.WriteLine(IsColor("red"));
           MyColors value;
           if (TryMatchColor("green", out value)) {
               Console.WriteLine(value);
           }
           if (TryMatchColor("turquoise", out value)) {
               Console.WriteLine(value);
           }
       }
       static bool IsColor(string name) {
           MyColors color;
           return TryMatchColor(name, out color);
       }
       static bool TryMatchColor(string name, out MyColors value) {
           try {
               value = (MyColors) Enum.Parse(typeof(MyColors), name, true);
               return true;
           } catch (ArgumentException) {
               value = 0; // default
               return false;
           }
       }

OK - reading again I may have missed your intention... forgetting about
solving it via Regex (or any other approach) - what exactly are you trying
to do? i.e. input {x}, output {y}...?

Marc

public bool isit(string c1)
{
    string[] color1 = new string[] {"blue", "green", "red"};
   return (Array.IndexOf<string>(color1, c1.ToLower()) > -1);
}