Changing HP Cartridge

[SteveSxx]

Why, whenever I change my colour ink cartridge on my HP PSC 500 is it never a simple job - the printer always refuses to accept it - I generally use "aftermarket" cartridges (cost 10 pounds as opposed to 30 pounds for genuine HP ones) yet I cannot get them to print (I get a message on the LCD display "REMOVE AND REPLACE COLOUR CARTRIDGE" - Which I do - no luck - I have cleaned the contacts (no colour ink emerges)
H - E - L - P !!!