Play 50 Wild Cards!
all you have to do is use a Wild Card (the ones with the circle divided into four colors) 50 times. they always stay in the left most side of your hand of cards. there are also achievements for picking each color (green and blue 5 times, yellow and red 20 times) once you play the card. this can be done over any number of games. check the in-game achievement list to see your progress.