You are the lucky owner of a printer that requires 3 cartridges of ink.
My first photo printer was like that too, but then I switched to the photosmart line of printers.
I pulled up your printer's manual online since it is no longer on the website.

#94 Black:
used for printing black only (basic text documents like shopping lists)

#97(16ml of ink) or #95 (11ml of ink) Color:
needed at all times (basic text documents with colored text)
used with photo ink for photo quality prints.

#99 Photo:
take out black ink and pop this ink into the printer to obtain photo quality.
It only works with the color cartridge in the printer.

Basically, for the color ink, just pick the one that is the cheapest per ml of ink. Your model was made at a time when both sizes of ink cartridges were available. I do see how this could all be confusing if this is your first HP printer, but I have owned this brand since the early days when they only came with black ink.

Oh... and when you take a cartridge out, put it in a plastic ziplock bag so it doesn't dry out (or pm me if it does for the trick on how to get it working again).

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Colleen in Friendly Manitoba...
Proudly selling digital designs at SBB
(Come take a peek at my Gallery or check my BLOG to see what I'm up to)