Copper deficiency. Make sure soil ph is between 6 and 7. Get some micro nutrients with copper and make sure you don’t over or underwater and the color should come right back.
Sometimes it is just over watering that can cause tips to turn white, then brown, like a nutrient burn. Are you using nutes on these nubies? Are the lights too close or hot? I have never really seen copper deficiencies from potted soil at this early in the game. Looks like you either used store bought or made your own mix. Which was it?
If it is copper deficiency, MDBuds, you got some good instincts there dude!
Make sure the water you are using is not too alkaline (7.5 range), that will lead to copper deficiency