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.
What is the purpose of this feeling? There is no obvious evolutionary advantage in "feeling" the 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?
Or, instead of the mind producing conscious experience could conscious experience be mind producing?