Day 4, after emerging. Is this coloration on the cotyledons okay?

Don't worry about the cotyledons, their only function is to protect the gowing tip as it pushes through the soil, after the seedling emerges, the cotyledons will die and wither away. They have no influence on the life of the cannabis plant after this. No need to feed if growing in decent soil and if growing in a reasonable sized potter for the first 2-3 weeks at least. She may be a bit of a slow starter, but have patience,.....although a 50w light is hardly gonna set the world on fire! All in all, your new baby plant is going to be just fine, but don't get too anxious "to do something", the plant will do everything it needs to get going all by itself. Happy gowing, and don't forget, if you have questions, ask them early, don't wait until it is a full blown disaster and before it turns into trying to rescue a near dead plant! People here love to offer help and love answering all questions, from the most basic to the most weird!!