With colors of light, the primaries are Red, Green, Blue. Green is not a mixture of other colors. To get yellow, you "mix" red and green. And no, blue can't go negative.
I am no expert on this, but I do know pigment colors (Cyan, Magenta, Yellow) are not the same as colors of light. I'm sure there will be an optics expert coming along soon to clarify.
Display RGB attempts to match the RGB cone response of the human eye. Much of what goes on to create "colors" on displays is, to some degree, trickery to fool the eye/brain into thinking that its getting a normal response, when its not.
Thus, "pure" yellow stimulates both R and G cones, by matching the same proportionality of response with R and G emissions from a display, the eye/brain gets similar stimulus and thus "sees" yellow.
Paulcet -- you were dead on. I was apparently suffering under the illusion that Red, Blue, and Yellow were the primary colors as the result of a massive fraud campaign perpetuated by K-6 art teachers everywhere. I'm still struggling with these new paradigms of RGB & CYM, they just seem unnatural to me after so much brainwashing during our formative years, despite the frequent references to RGB and CYM for computer monitors and printers respectively.
The following two links do a nice job of exposing the hoax: