To sign purple, make the ASL letter 'P' sign (a fist, extending the thumb and the middle finger, with the index finger extended and bent down ninety degrees). Take your 'P' sign and shake it around in front and to the side of your body.
Color signs are easy to remember because they are often the sign for the letter shaken slightly in front and to the side of your body.
You can teach the colors with more advanced signers by making a series of solid color flash cards and going through them while demonstrating the sign with pictures or actual objects within reach.
Using the purple flash card, together with either of the color Baby Sign flash cards, you can teach your infant how to recognize the colors. You want to keep this as one of the later activities, as color is a little more abstract concept.