Remember you must wash the bleach out of the hair thoroughly before applying the toner. You can also use violet tinted shampoo to tone the yellow out of your hair. If you want a permanent unnatural color, go to a salon and talk to a professional colorist and let them know what you are trying to achieve they should be able to give you the permanent shade you want. They are designed to fade. While there are things you can do to help the dye last longer, it is still possible for the dye to fade quickly.
Some people have better luck than others because of hair type.
Read over the dyeing section on this site for more information on what contributes to the lasting power of the dye in your hair. Basically, the surest way to keep the color is when it starts to fade recolor it. The answer to this question is primarily dependant upon your hair type, the current health of your hair, and the brand of products used. If you think your hair is in bad condition and you're worried about it, then don't do it - If your hair seems as healthy as ever, go ahead. The answer to this question is again primarily dependant upon your hair type, the current health of your hair, and the brand of products used.