I'm a licensed hair stylist, so I could't resist giving this science lesson >.> Prepare for a wall of text. That being said, here you go.
Manic Panic makes temporary dyes which are either semi-permanent (lasting 4 to 6 shampoos, potentially longer depending on the color you choose) or demi-permanent (lasting 4 to 6 weeks, again, maybe longer depending on the color).
Because semi and demi permanent dyes don't alter the natural pigment in your hair at all (rather, they sit on the outside of your hair, and are gradually washed away over time), you can't use them to make your hair any lighter; they can only darken your hair or add tone to your existing hair color. If you don't pre-lighten your hair before applying temporary dyes, the color won't be what is advertised on the bottle; it'll be that color mixed with your existing hair color, which will likely either muddy up the color, or prevent it from being visible at all if your hair is darker.
Keep in mind, however, that lightening your hair and applying high-pigment dyes like Manic Panic can cause the color you choose to hang around a lot longer, especially if you apply the dye immediately after lightening your hair. This is because lightener contains ammonia, which forces open the cuticle (the clear, scaly outermost protective layer of your hair that is responsible for your hair looking shiny) so that the peroxide in the developer can enter the cortex of the hair (where pigment is stored) and dissolve the natural or artificial pigment. Unless the pH of the hair is restored to normal, the cuticle of the hair can remain open for up to 3 days, so be sure to use a shampoo and conditioner meant for color treated hair. That will bring the pH of the hair back to normal, allow the cuticle to close and prevent the temporary dye from entering the cortex of your hair at all.
Also, if you choose to lighten your hair, keep in mind that different parts of your hair lighten at different rates. The roots of your hair lighten faster because the heat from your scalp accelerates the chemical reaction of the ammonia and peroxide. Because the ends of your hair are the oldest and typically most damaged part of your hair with the least in tact cuticle, they will also lighten faster. This means you'll want to start applying the lightener about half an inch to one inch away from your roots, and apply it down to the last inch to two inches of your ends. When you're almost to the degree of lightness you want, you can then apply the lightener to your roots and ends because by the time the midshaft is finished lightening, the roots and ends will have caught up and everything should be even.
Also, to create a pastel color out of a high pigment dye you really like, you can use a clear base to dilute it. The ratio of clear base to pigmented dye is up to you, but for really light colors, you'll only need a fraction of the amount of dye you'd usually use. I'd recommend doing a test strand first (take a small amount of hair, apply the dye, process for the recommended time, shampoo and observe, using a white towel as a background to make the color more visible), that way you can adjust the formula before applying it to a larger area.
I hope this helps if you're dead set on doing it yourself, though I ALWAYS recommend you let a professional do it because there are so many factors that can affect the way your hair color comes out in the end. And, if you're only dying a small area, you certainly won't be charged the same price as all over color. You can also bring in the dye you'd like to use, and have your stylist use it rather than whatever their salon carries.
I'd only recommend doing that for temporary dyes, however, as permanent dyes behave differently, and different brands of permanent dyes go by varying level systems and have different tonal characteristics (meaning that one brand's 6C/R (dark blond/ copper/ red) may look entirely different from another brand's 6C/R).