a little more info would help:

temperatures, frequency of watering, soil type, how often they are fed and with what ferts, lighting and how close/far from the plants, humidity throughout the day,. Anything else that may be pertinant.

ofhand, just looking at the pictures, I would be inclined to say they are being over watered, and or overfed. The purpeling could be from too low of temperatures.

Plants at that young age should not need any more fertilizer than what it gets from the soil. ( what soil are you using?)
Watering ONLY when the soil is dry, ( you can check by hefting the pot and checking if it weighs about what unwatered soil would feel like, that is it should feel light) better yet, don't water until the plants start to droop.

what is the ph of the water you are using? Are you using well or filtered water or chlorinated tap water?

The most common mistakes made by new growers are :
1) OVERWATERING
2) OVERFEEDING


WITHOUT THE AFFORMENTIONED INFO, A REAL DIAGNOSIS IS NEXT TO IMPOSSIBLE