Red is ineffable because it's a neuron that flips from nay to yea. Of course nobody can share your experience - they haven't got the neuron that plays the role of "yup, that's red alright". They've got their own.
If you haven't read it, esr had an interesting take on qualia a while back http://esr.ibiblio.org/?p=1192#more-1192 . His take is that qualia is the internal feeling of having a particular sensation. That is that the qualia of redness is the feeling that is internally associated with seeing red.
Apparently that is the "hard problem" of conciousness: How does the mind produce concious experience? The topic of this article is the "soft problem": Why is it useful to conciousness at all?
