No one sees envy as a color, so it's a silly example anyway. Synaesthesia isn't even seeing emotions as colors - it corresponds with perception, whether it be touch or sound. Some people try to explain emotions by means of metaphors, but that's different than saying envy is green as though it has physical properties like a green tennis ball. Plato basically thought that a concept actually is tangible, but claimed that things like truth and other concepts existed in another dimension, separate from reality as you know it. This is what Billy is arguing against, the notion that there is some "Moral Truth" to be grasped at that exists independent of your existence and is an attribute like the color red. By the sounds of it, unless we can prove to Billy that there is a Moral Detector in the same way there are Wavelength Detectors, morality is subjective. But this is impossible. Interesting enough, detecting wavelengths doesn't help you prove colors, either! There is so much more to discuss, including how your mind processes colors, illumination, and even if you know even have a concept of the particular color. I already offered a standard of morality to be accepted or refuted, but does have some form of a provable morality.
Also, on the standard I offer, there is no need for a list any more than there is a need for a list of every possible way for the sun not to rise tomorrow and deny each possibility to conclude that it will rise tomorrow. As in science or any other means to objectively come to conclusions, we use principles. As Jack was saying, this is a topic of epistemology. But with principles, it is possible to apply some set of generalizations/inductions to any situation upon being put into a situation. For instance, if one principle of yours is "honesty is a crucial way to find value in other people". Like any principle, it has to be proven, but once it is proven, you can apply it to any situation thrown at you. You may make an error, but the same is true in science. You will only discover that your reasoning was shoddy to develop your principle, or you need to clarify your principle. Scientists do this all the time, Newton's Laws weren't thrown out because his laws didn't really explain cosmic interactions between entire galaxies or two stars. The principles at work were clarified to apply to certain bodies of mass (I'm not a physicist, so I can't say much more). Morality can work in a similar way...