This game is like Simon Says, so you have to memorize the color and fret number of the game. However, playing Harmonics is strange difficult for some people to grasp; you have to touch the string at the fret, but the string cannot touch the fretboard. As soon as you strum, let go of the finger and let it ring. It may take practice, as it is one of the harder techniques in the game.
If this is too difficult, resort to the pause method mentioned at the beginning of this guide. It also may help to write down the sequence, pause the game, and prepare yourself.