DIAGNOSIS The printhead nozzles are almost certainly clogged. With tank printers, if you don't print regularly, the ink in the fine nozzles can dry up and block the flow.
FIX You need to run the printer's deep cleaning cycle. 1) On the printer's control panel, press the 'Settings' button (wrench icon). 2) Navigate to 'Maintenance' > 'Deep Cleaning'. 3) Select the color that looks bad, or 'All Colors' if everything's off. Press OK. This will use a bit of ink to flush the lines. Print a nozzle check pattern (under Maintenance > Print Nozzle Check) afterwards to see if it improved.
IF THAT DIDN'T WORK If one color is still missing after 2-3 deep cleanings, you might have an air bubble in the printhead or a more stubborn clog. Try the 'Printhead Alignment' utility in the Maintenance menu, as misalignment can also cause strange color blending. As a last resort, power off the printer for 12 hours—sometimes letting the ink sit in the head loosens things up.
HEADS UP Deep cleaning uses a lot of ink from those tanks. Don't run it more than twice in a row. If the problem isn't fixed, take a break and try again later to avoid wasting ink.