Well, if it is not by experience of a thing, we can know a thing, then I do not know how else can we know? Imagining how the thing should function?

As far as I can tell, "the hard problem of conciousness" is not a a fully accepted fact within neuroscience, as such, I will not comment on it. Though, since I take the stance of a determinist, I think the experience of the color red is shaped by how culture imbues symbolism onto a wavelenght, and previous positive or negative responses towards red things.