The change in the color may be from the formula. However, I think you don't need to worry too much, if he is happy and doensn't show signs of being in pain and gassy, which was your main concern to switch formulas, then it means that is working for him. The stool would be a concern if he was constipated or you could tell he is somewhat sick, but if is only the color, then in my opinion you shoudn't worry. Also, try to give this formula some time .
I totally relate to your fears and concerns specially now that this is your first baby, but remember to also trust your instincts. If he is happy, he eats well, is changed when needed, and sleeps well,then you just take a deep breath and sit back, have a cup of tea and relax. You'll know when something is wrong, beleive me,you will.
I also hope that the medicine is working and along with this new formula I am sure he is much happier.