Question about Canon PIXMA G3501 Megatank

YES, the printer has ink tanks into which you can fill ink. You don't need replacement cartridges as usual. I would be careful with the ink, though. Firstly, it's not as expensive as usual, and secondly, it's a problem with warranty claims if there's a defect.

Yes, there are paint containers. Yes, you can probably even fill them with water-based inks that you have mixed yourself. But at the price of the original sauce, I wouldn't bother - especially as the long ink lines & print head certainly won't work with every colour of water forever.