Poor quality in Printing when I used ink refill

My printer is HP Deskjet 3740 series.
I have two ink cartridges, the black cartridge and the primary colors cartridge. When the ink is used up, I just refill the ink cartridges with the refill inks (using syringes) which can be brought from computer stores. I followed the steps in how to refill. At first, the printing quality is really good.
But during the second refill, the printing quality isn't good anymore. The colors are mixed up, like when the color is supposedly yellow, it becomes red in printing. Now, I just brought new ink cartridges to replace the empty ones, which is really expensive compared to buying ink refills. I would really just like to refill my printer to save money.
I hope I can find answer here why my printer doesn't exactly shows the supposedly color result with the ink refills.
Thank you!
