I have a few friends that grow the purple and blues and was told that rather then using purple maxx they either drop the temp significantly the last 1-2 weeks or if thats not possible they simply grow over 1-1.5 weeks which also works. The 1 friend who grows purple cush told me that growing over nearly turns the entire plant purple.
It should work though seeing as most blue based plants had a purple plant in their genetic line to give them the blue. I have no clue what the ingredients are but was told first hand that the stuff does work.