My lemon balm leaves now have purple undersides, and several stems are purple now, too. There's some new leaves growing at the base of the plant which are a dark green color (no purple). Could the purpling be due to some sort of mineral deficiency/toxicity? Will it help if I prune it back to the dark green part?

This plant also had some issues with fungus gnats over the winter, which I combatted with a fungicide and severely cutting back watering. Perhaps the fungus is still at work?

Any tips would be greatly appreciated :)