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That is usually a problem that arises in one of two circumstances:
1. when the cartridge is quite simply, wrong for the specific model of printer (and thus won't fit into its cartridge bays)
2. when the cartridge is correct for the printer model, but, it is right for only one of the cartridge slots in the cartridge bay, and won't fit in the others.
Also perhaps worth mentioning, is when the cartridge bay or a slot in it, is damaged, and a abnormally protruding part block cartridges from being inserted.
What printer maker, and model #, is it?
And what is the color cartridge's type-number? (I would guess it is an HP printer, but I might be wrong, and it could be Lexmark, etc. Let me know, please. Thanks.)