Firstly, Mazal Tov on the birth of your baby boy. May you both see much nachas from him as he grows up.
I’m not up on the latest medical literature, so I don’t know if the current medical consensus is that a bris is (medically) safe if the baby is yellow or not. That aside, however, I would imagine that this scenario is very much that of terifos — in the times of Chazal, an animal was a teraifah if it had a defect that would cause it to die in less than 12 months. Nowadays, however, we see that animals with these defects DO manage to live longer. Nonetheless, the definition of a teraifah does not change — so even though an animal will live longer than 12 months, such animals are still ruled non-kosher.
Same thing here. Even though the child might be medically fit for a bris if he’s yellow, halachically, we rule that he isn’t medically fit.
All this aside, I would be shocked if any mohel did a bris on a baby that he considered to be unready for it despite the protestations of the mother or father.
The Wolf