ArsTecnica says: Yes, Virginia, there is a magenta.
It all depends on what you mean by "color." Wavelengths of light can be plotted on a continuous line, but the human eye perceives color with three different types of cone cells. The magic number here is three. Three points define a plane, so we perceive a color wheel The ends of the (1D) line wrap around into a (2D) circle.
Most of us have three types of cones, that is. Wikipedia says:
It has been suggested that as women have two different X chromosomes in their cells, some of them could be carrying some variant cone cell pigments, thereby being born as full tetrachromats and having four different simultaneously functioning kinds of cone cells, each type with a specific pattern of responsiveness to different wave lengths of light in the range of the visible spectrum. One study suggested that 2–3% of the world's women might have the kind of fourth cone that lies between the standard red and green cones, giving, theoretically, a significant increase in color differentiation. Another study suggests that as many as 50% of women and 8% of men may have four photopigments.I wonder if these folks perceive a color sphere, rather than a color wheel? (Since you need 4 points to define a sphere.)
Update, 24 June 2009: more here.