Don't forget sometimes shadow may not be gray at all. And most times shadow is dark. Most importantly the gray ink has nothing to do with color balance of the color photo. False information among daily forum posts is common. Just wanted to point that out. Any different thoughts are welcome.
By the way, Black IS one of the ink used in printing color photos. Why does Pro9900 use a CLI-8BK? It is for printing dark black shadow. If the shadow is supposed to have a tint it adds color ink to print it. The gray ink is for printing light gray tones in a photo. Honestly my Pro9000 prints light gray just fine without the gray ink. I will argue that it only helps when you print black and white photos.
In the world of pigment based photo inks, there is a need for gray ink because pigment black ink has a very small gamut. It alone is not able to reproduce light gray. But dye based ink is different. A dedicated gray does make a difference. The difference is not as significant as in the case of pigment based inks. The benefit of having a dye based gray ink is limited although can be appreciated. If you print black and whit photos you will appreciate it for not to worrying about color tint in gray.
By the way, Black IS one of the ink used in printing color photos. Why does Pro9900 use a CLI-8BK? It is for printing dark black shadow. If the shadow is supposed to have a tint it adds color ink to print it. The gray ink is for printing light gray tones in a photo. Honestly my Pro9000 prints light gray just fine without the gray ink. I will argue that it only helps when you print black and white photos.
In the world of pigment based photo inks, there is a need for gray ink because pigment black ink has a very small gamut. It alone is not able to reproduce light gray. But dye based ink is different. A dedicated gray does make a difference. The difference is not as significant as in the case of pigment based inks. The benefit of having a dye based gray ink is limited although can be appreciated. If you print black and whit photos you will appreciate it for not to worrying about color tint in gray.