As far as I can tell, that's an artefact of the development of the language, and not an inability to distinguish the two colours - colour-blindness appears to be no more prevalent in Japan than elsewhere.
It isn't that simple. For example, how many bands of colors are there in a rainbow? The answer to that question depends upon your language. Your eye may not be different from a person who speaks another language, and you both can actually discern about a million colors. But, you will still perceive the rainbow in bands of colors, and what those bands are depends in large part on your native language and its words for color.
So, now we have a problem when all we have is reports of a phenomenon. We know that people with different perspectives will see different things, and report different things. Sometimes, they will be looking at the same reality, and reporting different things. Sometimes they will be seeing different realities or phenomena, and reporting different things. And, we can't tell the two cases apart.
Now, for light, we have the recourse of stepping back and saying that it's all really electromagnetic radiation, and that the structures described in the rainbow are an artifact of language and perception, and have no reality - that is still oversimplified, because such perceptions have cultural impacts, but let's go with it for now. We don't have that for God. Even if we are looking at it all as merely historical mythology, the amount of cultural mixing and change going on leaves us with no clear and objective reality to fall back to like we do with light.
It comes down to a question that gets asked in other forms in other places: What does it mean to have an identity? How much change does a person (or a god) have to undergo before we recognize that we are not really talking about he same person any more?
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