Install photo cartridge -get 'Right cartridge missing'

The color cartridge ran out, so I tried replacing it with a photo cartridge, but no matter how often I tried it gave 'color cartridge missing'. No help on the website, so joined as a member. Browsing other similar posts, I eventually tried putting the photo cartridge in the B&W holder. Which worked. Eventually looked at the user guide, which shows this is what you are supposed to do.

What a weird system! Why arent they interchangeable slots, or at least have clear instructions on the cartridge saying how to install it?