I love bright colors & black clothes, but I don't like seeing them fade and get dull. What I do is on the first wash, I put the new garment in the washer with 1 cup of white vinegar. This sets the dye and prevents fading. You can but don't have to use detergent--it's all about setting the dye.
If you have a bunch of new clothes in similar colors, combine them, but otherwise, don't mix them in with your regular laundry. You want the white vinegar to concentrate on the new garment.