Phaser 7300 - one color fading on diagnostics page

dirty scanner.open the printer back cover.remove the scanner assembly clean it ,do the color calibration.u should be fine.

Amuna1

What you are describing is a bad print head. You can try running the eliminate light stripes several times, and jet substitution, but most likely the print head will have to be replaced to remedy your problem.
You can try googling. Phaser printer print heads or phaser printheads.
There seems to be a secondary market appearing for those print heads. I can tell you from Xerox, if you have no maintenance agreement, they cost $750 per print head.
Honestly, having worked on about every type of color printer known to man, the new solid ink phaser printers, produce a dry ink image of 2400 x 2400 dpi with ink particles that align end to end. The resolution of those printers are remarkable and can be purchased for less than the price of one of those new print heads.
Now for reliable, albeit somewhat not as sharp a color image, but still exceptional, you really can not beat an HP.
The ink stick technology really no longer has the color vividness advantage it initially had. The inks that align end to end, have ended that advantage and are sure a lot cheaper to operate per page.

If the issue is occuring in the same place on all prints, try the imaging drum (can't remember if this printer even uses one) or the transfer belt/roller (not sure which it uses). If you phone Dell support, they should run you through troubleshooting and send you either of those parts free of charge if you are under warranty. If not, buy them anyways; they don't degrade over time (if kept in the dark) and can be saved for when the currently installed one runs out if you don't need it to fix this issue.

cartridge must be expired

You need a cyan developer housing.

That really sounds like the imaging unit cleaner has an issue.
If you have one try the imaging unit, I really suspect that is your problem.
Black is Faded or Missing Step 1 Clean the Laser Windows and Remove Excess Toner From the Imaging Unit Step 2 Check or Replace the Imaging Unit Gear Retaining Pin Step 3 Replace the Imaging Unit (installation instructions)
See if this link works. Xerox is infameous for these not working.
http://office.support.xerox.com/SRVS/CGI-BIN/WEBCGI.cgi/,/?St=842,E=0000000000019251353,K=6024,Sxi=13,Case=obj(19377)

sounds like a bad cyan developer housing. Or that somehow the housing has come out of its mounting, but I cant think how that would happen. There are some rails in the machine that may have also come loose.
You have to remove the IBT ( transfer) assembly to look into that.