I guess their chest is more white than yellow now that you point it out. That chick looks fine. It looks like an Australorp! When growing their feathers out, they definitely go through what my wife and I call, their "awkward teen days". lol. Mine have gotten spotty white areas, ugly feathers and so forth but it doesn't take long for them to become good looking solid black birds. I don't know what you would worry about really anyway, unless you are planning to show them. To me, if they lay lots of eggs, they are perfect! I have lots of Australorps, Black Copper Marans, White Leghorns and a few other pure breeds...and now I have tons of barnyard mix that lay just great.