> what I call "red" could be very different to someone else's subjective perception

It's worth noting that is true of virtually everything we know. >>This is a very simple sentence.<< Anybody who understands English, 'understands' it. But what it means to understand it is perhaps completely different for each person. As long as they fit into the same place in their worldview (Lewis Caroll's Carrollian syllogisms come to mind), practically it often doesn't matter beyond recognizing the wonderful uniqueness of each human being. Likewise, unless somebody is color blind or perceives more colors than others (tetrachromats), it doesn't matter since the relationships between the different concepts or colors will be analogous amongst most people - so a common understanding within the differences is possible. Or perhaps it is more precise to say that there are so many data points in color perception or anything we know, that despite the minor differences in relationships, we understand each other because the differences must be minimal given the practically unlimited data points constraining our perceptions. In fact, when people's perceptions of things vary too much, they can be classified as mentally ill even if they understand many things perfectly well.