Harsh cleaners like bleach, toluene, acetone, etc. can harm the fabric by making it sticky, porous or yellow and should be considered cleaners of last resort as you may destroy your boat.
Your best bet is to try using Aurora Speed Clean with Hypexine. Hypexine is a leaching agent that will not damage the fabric. I would suggest rubbing the Speed Clean into the stained spots with a sponge or scrub brush and let it sit until it dries. Add more Speed Clean and give it a good scrub. You may wish to clean the rest of the boat at the same time. The next step is to expose the boat to sunlight. Over time it will bleach the stain out. This may take weeks or even months, but it won’t damage the boat.
During the sun bleach period, do not put any protection on the tubes and do not cover it. You can clean it periodically with Speed Clean. When the stain has faded to your satisfaction, apply 2 coats of Poly Guard to protect the fabric. Poly Guard will also protect the substrate against future staining from mold or mildew.
I would not recommend painting the inflatable. I have yet to see good results from painting PVC or Hypalon. The boat looks like it was painted and often will peel and flake. Not pretty.