Make sure that the thread you use is colorfast and will not fade into the white backing. I think the purple thread would be a nice contrast and would bring out the pattern of the quilting to enhance the overall design of the quilt. The ladies in my family were very traditional in their quilting methods. Nothing was ever machine quilted and all were both pieced and quilted by hand. I am sure that it would have been unheard of to them to quilt with a thread of any color; however I am more willing to see diversity in things than they were. If you feel that it will add to the overall beauty of your creation, go for it by all means. There is nothing wrong with being different as long as you don't do any harm. :-D