That kinda looks more like a problem then a color (jk really) i must be color blind i see brown not purple... Wanna give some details on the grow? Usually a browning/purple can happen in the fall, or what temps reach fairly low while depriving them of a certain nutrient. (Not going to say because thats just plant abuse to get a desired color)
I started my autos out in the water but due to pump failure I decided to put them in soil . I've always fed them on lite nutes but they came from the same company that sent me the bad pump they're on 18 6 cheap lights very inexpensive.. They were doing very well until my pump went out