How much are you watering it? How ofter? Nutes? How close are the lights? What kind of light? To me, It looks like a light might have been to close to the leave.
Look at her stems right now. Look at how they are purple, there is hint number 1. Also, check the underside of your leaves and look at the veins. I'm guessing that is where hint number 2 will be. She looks like a big girl that is maybe a month old or so? Read up on your sick plants, but it's probably micro nutrients. I'm not going to give you the answers because you'll learn more from reading the sick plants part .
I've read that purple is a characteristic of when temps drop below 70-60 degrees, or atleast it enhances the purple hue characteristics in some strains. What puzzles me is I'm week 1 into my first grow and one of my plants have rich deep purple leaf sets growing out the middle and the under side of the leaves are purple. It's sexy as hell and the plant doesn't look sick compared to its regular green fellow babies.