Hi Jane! Have you had a water analysis done? The color could be from minerals instead of biological activity.
Was the water yellow before the new well pump was installed? So I don't think ground water is not going to be sterile. I have a well, and I've read all sorts of information from industry and they recommend that the thing to do is "shock" a well with chlorine when certain problems arise. Pouring poison into the earth is narrow minded thinking and is a temporary band-aid fix that also has other unforeseen impacts and consequences. It makes no sense to me as it will never kill 100% of any microbe down there - there will always be some that survive and repopulate. I do not recommend pouring biocides into the earth, but instead establish what is causing the color, and if it is something that you can't live with, may I suggest alternatives such as a
carbon block filter, which may remove the color. Hope this helps!