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How can I get the black to seep through the tie dye?

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http://funadvice.com/r/15igm7a4gem
So im making these,i have the studs and blue and purple dye.
My initial plan was to use white shorts but i noticed that some areas are a lil darker like at the top they are a dark blue, would this effectnot be there if i used white shorts and if not how could i get that colour seeping through?