To me purple is the sign of stress, too many nutes, ph, heat and watering issues, can mean other things but i never had that. If the flushing helped one plant out it might be salt buildup in the soil or ph. When you say the perlite was brought up from the bottom do you mean that you watered the pot and the perlite in the whole pot floated? this could be bad for your roots. I don't think its an over fert problem and if one plant is getting better give its a start. If a soil flush helped i'd probably lean to a ph imbalance. See how the plants go after the flush. Not bad looking plants you got there.