The 9900 is more for doing proofing images for the final product (graphic design jobs), but if you do portraits, landscapes, etc, the Orange and Green may help with the skin tone, grass, leaves and anything else with those colors.
We've also done a test print on the 7900 with and w/o the 2 extra colors and really didn't see a difference. We did the test print on a flower with high contract greens and some orange tones.
If you're doing mainly photo prints and not much of proofing to final prints, I would recommend going with the 9890 as well.
Let us know if you have any other questions regarding these 2 models.
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