Hi,
Dyeing depends on the the fabric and the kind of dyes you're using. Different dyes are made for different types of fabric. You'll have to check to see if your dyes will work with the fabric your shorts are made of. I work with fiber reactive dyes which work on cotton, rayon, silk, wool, cotton/lycra blends and blends of each. When dyeing black I have to use higher temperatures, more dye and more agitation to get a solid black.
Hope this helps,
Jo