a subjective moralist saying "X is immoral" is like me saying "the blue M&M is green"

As I said, yes, by redefining every word then yes, you can make your statement true. Just as if I redefined "chair" to object and "not" to "certainly is", then I can say "This chair is not a chair". However, that doesn't change the problem still at hand. Moreover, in the example you gave where redefining some words, the difference between subjectivism and objectivism is still extremely clear.Green is an objectively true statement. If something is green, then it is green for everyone. Language may dispute it to a minor extent (and colour is an awkward issue to pick because it is an intuitively known thing, meaning it's impossible to define), but everyone knows what green is, and if you say something isn't green, we'd usually say you're wrong. This is to do with 'facts'.Morality, by contrast, is subjectively true (for a moral subjectivist). If something is wrong, it is wrong for a community and outside that community it may be right or wrong. Thus, there is a clear difference. My opponent may "resent a subjective definition for morality", but that's not my problem: its theirs. I dislike a deontological definition of morality. That doesn't stop it being an explanation of morality.As I have some space, I'll fill it up with different meta-ethical theories (explanations of what we mean when we say "X is wrong"Possible Moral Objectivist TheoriesKantian Morality - X conflicts with the categorical imperatives (it either cannot be universalised or uses people as a means to an end)Utilitarianism - X goes against the production of the greatest good for the greatest numberSituation Ethics - X goes against loveVirtue Ethics - X cultivates the wrong virtuesPossible Moral Subjectivist TheoriesContractualism - X breaches the contract made between individuals in a communityConsensualism - X goes against what people want to be done to them in a communityPrescriptivism - X goes against the imperatives people ascribe to one anotherNow, you may notice that some of them are not necessarily objective or subjective. The interesting thing about subjectivism is that it does not necessarily have a community where some are excluded. In other words, some of the theories may include every single individual when it comes to evaluation. This is a universal subjective theory.The most clear example is universal prescriptivism. It is an extremely useful theory in the sense that it has the benefit of holding everyone to a moral standard, but at the same time it doesn't need a moral code to exist objectively and inpdendent to us - it still remains an institution which isn't a moral fact but a social system. Morally objective theories are true for everyone, because they are objective and thus exist whether we do or not. Morally subjective theories are true for everyone, because it works for a universal community, but stop existing if we do not.Consider currency. Money does not have an objective worth. If we stop existing, the dollar won't be worth anything. It has no objective value. However, it is universally valued, and as such it is worth something to everyone, and bound to a monetary value, whether we as an individual want it to or not.Thus, subjective morality is for the community, and thus does not exist outside a community. By contrast, things like the colour of objects exist outside of the community, and thus are a certain colour whether we like it or not. Both of these are valid moral hypotheses, but they just relate to the world differently. Moreover, subjective morality isn't necessarily preference. Many subjectivist theories work in such a way that what society likes is irrelevant: contractualism is a good example of this. It is an ethical theory which says morality is the acceptance of contracts by two parties, meaning two individuals accept a common morality. However, the end result is the same: morality works as long as people are around, while the colour of objects is true regardless of preference; it is true whether we like it or not.