Rainbow-colored tie-dye projects start with white garments, but you can create a different effect by using bleach to "dye" dark-colored shirts that might otherwise end up in the donation bin. The longer you leave the bleach on, the lighter the sections become, but you need to prepare the shirt meticulously to make sure the bleach doesn't bleed beyond the places you want it.
Dark colors like black and navy blue work best with bleach as your tie-dye medium. The bleach does not immediately turn the fabric white, so you can experiment with different color gradations. Bleach has a powerful smell and lightens any fabrics it happens to splatter on, so it's best to do this project outside or in an open, well ventilated work area such as the laundry room or garage. Cover or remove from the area anything you don't want bleached, and wear old work clothes. Put on rubber gloves to protect your hands and a painter's mask so you don't inhale the fumes. You can dip the shirt in a tub of bleach or pour the bleach into a spray bottle to keep it under control. Have a drying area ready as well, and include an old towel to absorb excess bleach. After the bleach dries, wash the shirt before you wear it to get rid of the caustic smell.
Rubber bands are your main tool to use when tie-dyeing; they block off the areas that won't change color. The twists and turns you create in the shirt before securing it with the bands also determine the final design. For a classic sunburst tie-dye design, start with a wooden dowel. Place it in the middle of the shirt, and twist the shirt tightly around the dowel. Secure the shirt in this twisted swirl with rubber bands and remove the dowel. If you want a striped effect, gather the shirt together like an accordion -- don't roll it -- length- or width-wise, and wrap rubber bands around it in a line at regular intervals. Dip the shirt in a tub of bleach or spray it with bleach for a more abstract pattern. If you dip the shirt, roll it in the towel before you remove the rubber bands; set it flat on a rack to dry.
Once you're comfortable with the technique, you can get more creative. Before bleaching, gather small sections of the shirt here and there and wrap them with rubber bands to make tie-dye circle shapes, scattered or in a pattern, or twist the shirt up and wrap rubber bands around random areas. You can dispense with the rubber bands altogether and lay shapes, such as plastic star or heart cookie cutters or wooden square blocks, over the shirt. Spray the bleach on or splatter it over the shirt with an old paintbrush. Use objects that can stand up to the bleach.
Experiment on scrap fabric to refine your artistic technique, and then spray the bleach in a fine mist directly on the shirt as it lies flat to create a splatter effect. This works especially well if you aim the spray up so it falls in an arc onto the shirt. You can also use an old paintbrush for thicker, more defined splatters or use both techniques together.