I have changed the colorcartridge and now it does not print the color I want. Blue changed into red. How can I fix this? By the way, my printer is a HP 5940 Deskjet

You may have put the wrong color cartridge in the wrong print cartridge position.

SOLUTION Mostly a new genuine cartridge from HP does not have such problem , but you can try cleaning the heads by using the head cleaning utility in your printer software. or you can rub a few drops of isopropryl alcohol on the print head,(that is available on local drug store) and run the head cleaning utility.
WHY THIS OCCURS This problem occurs either if the ink tank is going to be empty, or using a refilled ink cartridge, or you print head is jammed, or the ink cartridge is defective.
KNOW HOW DOES THE PRINTER PRINTS The inkjet printer prints by spraying the required amount of inks from the numerous and try print head nozzles, it uses three colors for printing color prints i.e, CYAN, MAGENTA & YELLOW, clean nozzles means the perfect result, now if the CYAN nozzle is blocked then the printer cannot fire the blue ink and the result is that the blue places in the printout is changed into red, if some of nozzles of all three colors or one or two is blocked then you will have horizontal lines all over the print out.