DIAGNOSIS The print head nozzles are clogged, which is super common if the printer hasn't been used for a few weeks. New ink is in the cartridges, but it can't get through the tiny holes in the print head.
FIX You need to run the printer's cleaning cycles. From the home screen on the control panel, touch Settings (the wrench icon). Go to Maintenance > Print Head Cleaning. Run the Standard cleaning first. Print a test page (there's an option right in that menu). If still faded, run a second Standard cleaning. Avoid using the Deep Cleaning option unless absolutely necessary—it uses a ton of ink.
IF THAT DIDN'T WORK The ink cartridges themselves might not be seated properly or the protective tape wasn't removed. Turn the printer off, open the front cover, and firmly press each cartridge down until you feel a solid click. Also, double-check you removed the orange protective tape from the bottom of each new cartridge.
HEADS UP These inkjets don't like sitting idle. If you print even one colorful page a week, it keeps the ink flowing. If you go on vacation, run a quick print job when you get back before you see a problem.