sorry to say guys but there isnt a such thing as "purple" frontosas..
http://www.cyphos.com/forums/showthread.php?t=19487
also if your wondering how do you tell the difference between the frontosas..go to this link..its all in the mask and color..
http://www.cyphos.com/forums/showthread.php?t=11854
hope this help..but to answer your question..if your going to go with a tank of frontosas..mightest well go all out and go Zaires..(kitumbas, mobas, kapampas..)..but if you dont want to spend too much then go with the northern type..(Burundi, Kitgomas(7-stripes))...best of luck with your decision..and remember..a colony of frontosas need at least a 6foot long tank..