I don't really know about the purple tip needles so I googled them. One of the sites I found said they were good for sewing on stretch fabrics so maybe they are a ball point needle?
Edit: I just found this.
Red Tip Needle - Has a normal tip. Designed for regular sewing on fabrics such as cotton.
Blue Tip Needle - Has a ball point. Designed for sewing on knits and synthetic fabric.
Purple Tip Needle - Has a ball point with a special "cobra" head. Recommended for high density designs and thick fabrics.
I prefer a Microtex needle for most of the sewing and piecing I do.