The 24 range includes light cyan and light magenta in addition to black, cyan, magenta and yellow. These are photo-oriented Expression printers and the extra inks give smoother tonal transitions. All six must be fitted for the printer to work.

They fit exactly the same printers — the XL simply holds more ink. If you print often, 24XL works out cheaper per page and means fewer cartridge changes. For light use the standard 24 may be enough. You can run one of each in the same printer without any problem.

Yes. The 24XL range uses separate cartridges for each colour, so you only replace the one that has actually run out. A multipack normally works out cheaper than buying four singles, so it's worth a full set if you print regularly in colour.

Keep them sealed in their packaging, upright, somewhere cool and out of direct sunlight. Unopened cartridges keep well for a long time; once the foil is opened the ink can begin to dry, so only unwrap one when you're ready to fit it.