[phpnode]

it'll tell you that `string` cannot be assigned to type `"RED" | "GREEN"`, and so you'll need to write a function that refines the type correctly, e.g.

    function isValidInput(input: unknown): input is "RED" | "GREEN" {
      return input === "RED" || input === "GREEN";
    }

    const input = readFromUser();
    if (isValidInput(input)) {
      doTheThing(input); // no type error
    }