I'm embarrassed to even ask, but...

var simpleArray = ['red', 'green', 'blue'] // doesn't get easier than this

var message = ''; // store empty value in message variable to avoid "undefined" errors. I don't think I'm interacting with this variable at all, but I included it here anyway.

// pre-formats the print message
function print(message) {
  var outputDiv = document.getElementById('output');
  outputDiv.innerHTML = message;
}

function runProgram(){ // start program with no parameters. Not clear why I need to start a program here, other than "tell the JavaScript interpreter to do something." Doesn't calling the print function "do something?" 
  var input = prompt("Name one of the RGB values: "); // prompt user and store response in 'input' variable
  for (var i = 0; i < simpleArray; i += 1) { // cycle through the simple array
    if (input === simpleArray[i]) { // check the input of user against each array string value
      print("Yep, " + input + " is one of those values!"); // prints if the condition is true
    } else {
      print("Nope, try again.");
    } // ends else statement
  } // ends for loop
} // ends function

runProgram(); // runs program