Yah, it's not passing it at the same time, but it's additive because a lighter color on the wheel flashing after a primary is a sequenced addition of light to the previous saturation flash, or that's how I understood it. The other alternative method is to create a color wheel that is just a "brighter" mix with the primaries at certain points when combined with the yellow (or even other colors), hence a different way of mixing. Yellow is fairly close to the color of neutral light, even if white is more neutral, so my understanding is the Yellow segment in RGBRGBy is nothing more than an altered white segment (improved variation?), but I don't know for sure. Even if it does not function like a white segment, I do know it adds brightness either way, because Sharp added RGBY as a new mix into one of their TV's and it was a lot brighter (Although people weren't sure why Sharp really bothered since TV's were bright enough already). Although I've seen different claims of what the Viewsonic CW really is, one site suggest (RGBRYCB), but another site suggests (RGBRGBy). This yellow segment increasing brightness would make sense because the Viewsonic is like 1400 lumens in a "best mode".
5x is 25% faster, and 6x is 50% faster by RPM's than a 4x if the number of primary segments are the same.
A 4x color wheel is still 7200rpm, but they call it 4x's because of the number of primary segments are doubled when it is a 6-segment. So they already factor the segment calculations into the speed itself when talking about primary segments. They cannot get past 6x at a reasonable price level because the motors become too expensive and the speed and friction would be too great for the parts reliability to keep up, as well as I'm guessing eventually the DLP couldn't keep up. A color wheel is run by the same type of motor as a hard drive.
Here is how a color wheel works from PJC's explanation:
This wheel spins between the lamp and the DLP chip and alternates the color of the light hitting the chip from red to green to blue. The mirrors tilt away from or into the lens path based upon how much of each color is required for each pixel at any given moment in time. This activity modulates the light and produces the image that is projected onto the screen.
Today, many DLP projectors being built for the home theater market incorporate a six-segment color wheel which has two sequences of red, green, and blue. This wheel still spins at 7200 RPM, but because the red, green, and blue is refreshed twice in every rotation rather than once, the industry refers to this as a 4x rotation speed.