HP Envy Photo 7134 - Magenta-toned images

Hi, I have an HP Envy Photo 7134 with newish ink cartridges, and the test print comes out perfectly with all colours - CMYKB - coming out properly. However, although text comes out solid black, any images I print come out with a heavy magenta tone, as though the blue or yellow isn't there. This is a very weird issue I've not come across before. I don't really see how, if the test print is fine, images aren't. Nozzles are not blocked. I am using third party crtridges but the printer happily accepted them and they have been working fine until now. Does anyone have any idea what might be wrong here?