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
I second mike. On lil ones like that it’s too wet or nutrient lock from high PH 
. Let them feel light between watering or increase air flow (fan) for more transpiration. Test your water before watering. If it’s not above 7.4 then test the effluent at the bottom (run off). Next time it’s dry over water a little. Catch it in a bucket then tilt it on a shim. Sediment will settle at the deep end after a bit and you can pipet a clean sample from the shallow side. New soil can sometimes have too much alkaline buffer if the soil batches aren’t mixed enough by the bagger or home maker. Especially if lime was your primary buffer. I like to use aragonite sand (crushed coral) for my mix instead of lime. It releases slower.
I had not thought about turning up fan to promote transpiration. Will have to try that next time i am trying to address a ph problem. @