I would try flushing again, for a long period of time. It sounds like this was drilled mud rotarty, and if that's the case, what you are seeing could still be trace drilling fluid (which gets turned gray from gray clay formations). I drill mud rotary and install screened gravel packed wells, and this is a problem sometime when the well makes a lot of water or the driller doesn't spend enough development time. For example, I may install a well that is capable of pumping 200 gpm, and I airlift develop at only 100 gpm due to friction loss and other constraints of the airlift,then I put a 10 gpm pump in and hook it to a house. There may be large sections of screen that don't get developed, since the capacity of the well is so high, and over time, the drilling fluid is slowly broken down and tints the water. There are a couple solutions; heavy, heavy flushing, days or weeks at the highest rate you can. If this doesn't work, pulling the pump and re-developing the well by airlift or jet cleaning should get it. I have also seen this a problem if the driller didn't grout the well properly or gravel pack the screen properly. As far as the sulphur, we have that here too, and it seems to come and go for no reason at all. A confined aquifer is not affected by drought, so the water quality changes you are seeing are coincidental.