a couple things you nmeed to know abut PD's.....
They are picky and will apper brown until you find the happy spot in yur tank for light. I would generally agree with what has been said in concern to light, but everybodys system is different. i would start there, and give them time....adjust as needed.
As well, if you are expecting a super deep bright purple in comparison to somebodies over manipulated pics, or looking at a top down shot, you will never be able to match that deep of purple with them, but they can certainy get very purple just the same