My HP LaserJet 2840 is on a wireless network and I use my laptop to print to it. My laptop is Windows Vista 32 bit. Every time I print in color, I either get blank pages, a page with a little magenta, or a little cyan. I can print in black and white no problems. I can't tell if it would be something with the image drum or software problem.

SOURCE: can not locate and download 64 bit version of HP 2840 laserjet printer driver

Try below link, it may be able to help resolve you problem...
http://etech.maiila.com/hp-color-laserjet-2800-all-in-one-printer-series-driver-manual-and-software-download-including-for-vista

Good Luck

SOURCE: HP Laserjet 1500 Color, Print out Blank

Hi devi960:
Pages print, but all
colors do not print or
the pages are
completely blank.
One of the print cartridges might be
low or empty.
Run an engine test and see if all four colors print. If
not all colors print, replace one or more of the print cartridges. If the
engine test is successful, replace the formatter.
If the printed pages are completely
blank, the file might have blank
pages.
Check the file to make sure that it does not contain blank pages.
A print cartridge might be low or
empty.
Check the printer control panel for a Supplies Status message,
print a Supplies Status page, or use the embedded Web server
(HP color LaserJet 2500 series printers only) to see the levels
remaining in the print cartridges. Replace any print cartridges that
are low or empty. Messages are not created for non-HP print
cartridges.
If you are using non-HP print
cartridges, one or more print
cartridges are low or empty. The
control panel lights do not indicate
low or empty for non-HP print
cartridges.
Replace one or more of the print cartridges.
Font DIMM might be selected but the
font DIMM is not physically installed in
the printer. (This issue applies only to
HP color LaserJet 2500 series
printers using an Asian-language font
DIMM.)
In the settings for this printer, clear the Font DIMM check box on
the Configure tab if no font DIMM is installed in the printer. Select
the Font DIMM check box only if the font DIMM is physically
installed in the printer. If you think that the font DIMM is installed,
make sure that it is installed correctly.
The paper curls when
exiting the top output
bin.
The paper needs to print to the
straight-through paper path.
Open the rear output door to print to the straight-through paper
path. Remove each sheet as it exits the printer.
The stack was slightly curled when it
was placed in the tray.
Turn the stack over in the tray.
The paper does not meet
specifications.
See the user guide for information about print-media specifications.
If you are printing on light media, the
Type setting might not be correct.
Select Light in the Type is field in the printer driver.

SOURCE: HP 2840 Prints Blank Pages

Black is used on all prints. Did you pull the tape on the new cartridge?

SOURCE: HP 2840 10.1004 SUPPLY ERROR

there is something wrong with your drum.. try to replace a new chip for the drum make sure that the chip is working.... if that did not work. see if there is any damage in your drum.. If not replace a new drum..

SOURCE: We use HP Color Laserjet CM1017; Magenta color is not printing

Dirty laser unit - needs (careful) cleaning internally.

See ' Printing Weak Magenta', 'Printing Light Magenta' for HP CM1015, HP Color Laserjet 1600, 2600, 2605.

All the same laser unit