I painted my cast iron tub a few months ago with oil-based exterior
white enamel paint. It looked great at first, but now it's starting to
turn more of a cream color.

Thinking it was just dirty, I hit it with some "Soft Scrub with bleach"
and it didn't help. Is the paint yellowing, or is it something in my
water? I have old galvanized pipes through the house and it is a bit
rusty, but all the paint seems to all be the same cream color, even
areas that aren't ever directly exposed to water, only to steam.

If it is the water, is there some sort of urethane clear coat or
something I can spray or roll on that will hold up to the heat, water
and chemicals?