Get some more light on them and stick to just water until they get some time under the extra light.
A lot of strains I grow get red/purple stems and grow just great. Seems to happen more often as they get into maturity and are ready to flower.
By light watering I hope you don't mean small amounts of water often. Pot loves a wet/dry cycle so should always be watered to the point of runoff with pure water and allowed to run out about 20% extra when using harder tap water to flush excess minerals out enough so it doesn't build up in the pots.
Then let it get dry enough that the pot feels really light but the plant isn't drooping yet or maybe just beginning to droop. Then you know what it feels like to heft the pot and know when it needs a good watering.
Give it a good week to see how the plant reacts after you add more light. Wait and see works better than add more shit and pray.
Unless it's something critical and plant death looks imminent of course.