But they aren't the same.
They are the same if you take away our scientific understanding of color.
From a position of scientific naivety, I would be inclined to regard color perception as being directed at some real feature of the external world, but I would do so because was can have, say, two otherwise identical balls, viewed under identical conditions, and see one as red and the other as blue, and other people would reliably report their colors the same way I do. This would seem to deny the claim that color is merely conventional any footing--there's just nothing else in our experience to hang it on.
But you can't have two "otherwise identical balls" that are different colors under the same conditions. If you could, that would be an incredibly perplexing thing for us to understand. In fact, their surface composition has to be different or there has to be some similar difference. These differences are perceptible by other senses.
Moral intuitions are exactly not like this. We never have two otherwise identical acts such that one is right and the other wrong.
Imagine if we could have two otherwise identical acts such that one is accurately judged right and the other is accurately judged wrong. That would prove morality is subjective because here is nothing objective the judgment could measure.
If you had some kind of 'proxy' you could pass vision through. of course something different would have to pass through the proxy to see red rather than blue. If you could feed vision through such a proxy, and the same input produced different colors seen, that would *refute* objectivity.
Moral assessments do generally pass through such a proxy. I speak the thing you judge. So of course, I have to speak something different for you to judge something different. Were it not so, morality could not be measuring objective properties -- there must be some difference to measure.
In fact, because we can test morality under circumstances where we completely control the input, it's much easier to argue that the assessment is of objective properties. We can actually see that a person is assessing two different inputs, with objective differences that might be what they're measuring, when they produce different output.
We sweep whole classes of acts into one moral category or the other. We even completely disregard details that we don't consider relevant--it's generally taken to be a desirable feature of a moral system that its prescriptions be universalizable. And it's trivial to propose a mechanism to explain this without reference to some set of mind-independent facts about the world: maybe values are just transmitted from one generation to the next. Maybe they're ingrained in our biology. It doesn't matter in any case--none of this will get you to exotic entities like moral facts with normative force that provide logical grounds for moving from is to ought.
But this is the same about color. For example, the fact that we consider a mix of pure yellow and pure blue to be the same color as green is due to facts about our color vision. Two green balls can emit spectra of light that are as difference as a blue ball and a yellow ball, yet we see them as the same color because of facts ingrained in our biology.
We disregard all kinds of things when we assess an object's color. We zoom in on those things that relate to color, just as when making moral assessments, we zoom in on things that relate to morality.
Color names are transmitted from one generation to the next too. We draw the line between red and orange at a point that's learned, not necessarily part of biology. But whatever we learn about morality, it must be that we are learning how to assess some property. The question is what exactly is that property.