Stems turning white on pinus aristata

Hi there I am new to Bonzai‘s. My wife and I have started bonsai trees this year because we are having a new baby girl and we want the trees to be the same age that she is as she grows up. I currently have three sprouts of pinus Aristada and have noticed one of the sprouts stems is changing color toward the bottom from brown to a light gray or white instead of green. Is this normal and can anyone explain trimming the taproot ?

Am not totally sure what’s going on - can you share a photo?

As for the pruning the roots, I’d hold off on that until the tree gets a bit bigger.