Don't transplant auto's if you can help it. 3gal will be fine for it's whole life. It's a good idea to start in the final pot. Just go easy on the watering first couple of weeks until its root system is fully established. yes the plant needs light the moment it breaks the surface of the soil. get light on it ASAP. What lights are you going to be using? Have it on a 20/4 time schedule, which is best for Autos. Purple stems usually sort themselves out within 7-10 days, it's usually down to a colder environment (which I suspect, as she has been in the dark, the light will warm her up a bit) or lack of nutrients, but she won't need feeding for first week. In coco you will then want to start adding nutrients after the first 7 days at 1/8th or 1/4 strength as there are no pre-exisiting nutrients in coco. But beware - nutrient burn can stunt growth and everyday matters with autos. good luck
