We have several HP 2600N printers in our office and all are starting to fade when printing magenta color. The cartridges are not low and we have moved cartridges around and replaced with new without result.

Check out black ink level? If good do a hard reset of printer along with a software reinstall. If that does not resolve the problem you may have a defective print head or just a bad
black cartridge since on may HP units the print head is built into the cartridge.

I guess you should try to reset it ..and get the cartridge out and see if the toner (ink ) is getting out ..by pressing it (with the part where you removed the tape ) on some absorbent paper ..when it is working well it should leave a mark on the paper ..the mark should be not washed ...but real color and all...
If it do not ..just use a syringe and blow air in the cartridge ..just to agitate it ..and make sure it is not dry ..
Insert it back ..repeat the procedure until works ..and also if you have the procedure ..reset it ..
http://www.ehow.com/how_6881778_reset-2600n-toner-chip.html

Change the cartridge Magenta and try again.

Hello
The color of HP LJ 2600n fades due to following reasons
1. the color cartridge near to empty 2. due to dust on reflection mirrors inside the printer
if the color cartridges are OK, then you need help of HP technician support, because cleaning of mirrors is very technical in nature for 2600n.
Dusty Environments can lead to Laser Scanner Contamination. The dust layer on the mirrors prevents proper transmission of the laser output in the imaging system leading to faded colors on the printed page. The initial stages of this failure typically occur in the magenta or yellow color planes as faded or light color. Often the left side is more faded than the middle.
You may need to contact the nearest Authorized Service Tech and get teh Laser Scanner replaced with the new released service part that has a mirror in a location where the dust ideally cannot reach easily.
Also, please ensure that the printer is relocated from the current location to a more dust free zone.
Hope this helps, if so do rate the solution

A faded magenta on this model is probably a dirty scanner mirror.
The printer design puts a fan in an area that blows magenta toner directly at the laser scanner.
The more toner on the mirror, the more faded the print.
You will need to remove the scanner and clean it.

This is a common problem with this model of printer. The scanner optic degenration is due to dust buildup on mirrors and lenses. Since the most vulnerable mirror reflects its laser beam to the magenta toner cartridge, red tones are the first to show fading. Colours shift: purples turn blue and oranges turn yellow. The printer will need to be cleaned by a certified HP technician to resolve this issue as it means dismantling your printer to fix the problem.

you need to take the scanner out (not very easy to do)unless you are a printer tech.
and clean the mirrors.

This happens due to Laser sCANNER Contamination.

Light Output that typically starts fading along the outside edges of the media. The first noticable color would be Magenta.

Dusty Environments can lead to Laser Scanner Contamination. The dust layer on the mirrors prevents proper transmission of the laser output in the imaging system leading to faded colors on the printed page. The initial stages of this failure typically occur in the magenta or yellow color planes as faded or light color. Often the left side is more faded than the middle.

You may need to contact the nearest Authorized Service Tech and get teh Laser Scanner replaced with the new released service part that has a mirror in a location where the dust ideally cannot reach eazily.

Also, please ensure that the printer is relocated from the current location to a more dust free zone.

Common Problem!  You can fix this by taking machine apart and cleaning the mirrors and lasers.  Remove the toner cartridges and look inside--notice that the magenta cartridge sits right next to the fan.  Its a problem just waiting to happen.

This is caused by toner contamination in the laser scanner unit, it is quite a complex job removing and cleaning the laser scanner. I would suggest you contact HP regarding the repair.