Diagnosis
The printer's ink level detection is confused, usually because the ink absorber or waste tank counter is getting full. On these MegaTanks, the software tracks waste ink and will start blocking colors when it thinks it's near capacity, regardless of the actual tank levels.
How to Fix It
You need to reset the waste ink counter. With the printer on, hold down the Stop button for about 10 seconds until the Alarm lamp flashes 15 times. Release it, then press the Stop button once more. The printer will restart and the ink levels should reappear correctly.
If That Didn't Work
The other trigger is the actual ink absorber pad inside being saturated. If the reset works but the error comes back quickly, the pad likely needs replacing. That's a more involved disassembly, but kits are available online.
Heads Up
This reset trick is a temporary fix. The waste ink tank is internal, so if you do a ton of printing, especially cleaning cycles, it will eventually fill for real and require physical maintenance.