Are you sure about that? Do you think the meaning of "bluep" could be conveyed to young humans without having blue objects to point at and without those humans at least forming a concept of blue?

Who modded this up? Is this the standard now for what words a language could have? Whether the concept could be explained to a child? You might as well dismiss the word "oxygen" on the grounds that you can't explain the full chemical model to a child, in a way that allows you to make use of the concept.

In any case, you could explain it to a child: "When you're dreaming about blue, you're just blueping. But when you see blue for real, you're blueping and seeing something blue."

Why is it that "It's impossible to explain ..." always seems to say so much more about the speaker than the concept?