What's Wrong With My Printer?

I have an HP PSC 2355 printer that uses colour 344 and black 338 cartridges. It was working fine till I bought a replacement colour cartridge. I have tried to align it several times but the alignment is still off. The print (words and picture) come out with a grey shadow / outline behind each letter / picture. I have checked the settings and the font attributes, cleaned the cartridges and all seems to be well. What's wrong with my printer?