If anyone has a shit colored duo that they are willing to experiment on, I have a suggestion that may help. You see your duo and you want it painted black? First, go to a hardware or plumbing supply store and acquire some PVC primer. It is formulated to take the smooth slickness off of PVC (plastic) pipe and fittings before gluing them together, as well as any dirt. This stuff would probably take the yellowy shit right off of your duo, as well as prep it better for a paint job, should you decide to go KITT on your duo's ass. I would recommend spot testing on the bottom somewhere, possibly with a diluted solution. Even 25% primer may be enough, it's really strong stuff, so you will probably want to open a window as well. This stuff takes the printing off of PVC pipe, but it obviously doesn't harm the plastic. I seriously think it would work, but like I said try diluting it very weak with water first and if it doesn't do anything, add a little more primer to the mix and try again till you get the right results.