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

I've done this debate already using larger words, and realise that this was the wrong way of going about it. So instead I'll try and put forth a concise case.Firstly, those believing in objective morality believe that morality is binding to everyone, and that is the key aspect (though to be fair it is larger than this, it means it is independent of people, the fact that all are bound to morality is the key aspect). By contrast, ethical subjectivists say that morality is binding to communities. Outside of the community, the ethics is no longer binding.This is a debate in the subject known as meta-ethics, meaning the study of ethics. Meta-ethics is the deal of saying "what do I mean when I say Murder is wrong?" So, applying what we know, people who believe in objective morality say that everyone has to follow this moral system, and by contrast those who believe in subjective morality says everyone inside this community has to follow this moral system. An example helps. If a 20th century evangelist was brought into 14th century Maya where a sacrifice ritual was taking place, then a moral objectivist would argue (in general) that killing the individual is wrong, as there is an objective law by God telling us this is wrong. A moral relativist would say, by contrast, would say that this is morally right in their community, but morally wrong in his/her. Or give another example. Imagine I live in Taliban Afghanistan. A moral objectivist there would say their Sharia Law is objectively morally right, and works everywhere, should be in place everywhere, and anyone disagreeing is wrong. A moral subjectivist would say that in their community, they may think they're right, but a Western individual would say that their morality is wrong.To sum up, the moral law of an objectivist is universal and applies to every individual, regardless of their communities. By contrast, the moral law of a subjectivist is binding to a community, and as such does not apply to people outside the community.How does this make moral subjectivists different from moral nihilists though? Well, imagine I'm a muslim in Taliban Afghanistan. A nihilist would say that a woman nor wearing a burkha or a man shaving is neither morally right nor wrong. A moral objectivist would claim that, no matter if they were in Afghanistan or India or Australia or Venus, people should still be muslim, women should wear the burkha and men should not shave. Moral subjectivists claim something different though. They say that yes, one should be muslim in Taliban Afghanistan, but in Australia, the morality is different. The moral subjectivist may disagree with whether shaving is moral or not on a preference level, but inside the community, one should, by being moral, not shave. Morality becomes essentially conforming with expectations.So we have a solution to the meta-ethical problem. When we say Murder is wrong, while a moral objectivist says murder conflicts with a moral law or principle, and nihilists may mean murder is neither permissible nor impermissible, moral subjectivists say murder is an act that is abnormal to the community, and diverges from communal customs. My opponent's case is that "when subjective moralists say "rape is immoral", all they mean is "i dislike rape" or "society dislikes rape" or something along those lines". And he is right, that some claim this. To be fair, though, clearly the first person pronoun and the third person "society" makes a massive difference in what morality is. And yes, if we start redefining "green", "blue", and "M&M", then we can claim "blue M&Ms are green". However, there is a massive difference. The fact that an M&M is green is a factual issue, independent of our opinion. It relates to the correspondence theory of truth, a posh way of saying "things are what they are". Morality relates to the coherence theory of truth, which means "things are how we see them". The former claims a physical fact, while the other is a social institution.