hydro noob/root question

so I'm a hydro noob, I'm reading and learning what I can but I didn't find anything on this particular subject
I just started my first hydro grow and everything's going great roots just popped through the bottom of the rockwool and all but I noticed the very tip of some of the roots are a little darker than the rest and I was curious is this is normal or if there is a problem..
thanks for reading, hope someone can help

also.. I suppose I can start nutrients soon?

The darkening of the root tips could possibly be a sign of the early stages of nute burn. Are you feeding nutes right now? If so what is your ppm? How many leaf sets do your ladies have?

no nutrients right now, they have 2 sets of leaves (I was using nutrients at 1/4 solution for a few days til I realized I shouldn't be and switched the water to straight distilled)