Eyes Test Cards

Effect:A four of a kind is shown,one at a time these cards turn face down. The performer asks the color of the backs of the cards which were shown only a moment ago to be red,only to find they've now all turned blue.The question is asked again and what ever the performer replies you state that he/she must need their eyes tested because when the cards are shown once again their backs have now been replaced with miniature eye charts. A simple direct trick which is easy