Rainbow-colored tie-dye projects start with white garments, but it's possible to work with dark-colored shirts, too. The secret is bleach. Dark colors like black and navy blue work best. The bleach doesn't immediately turn the fabric white, so you can experiment with different color gradations. The longer you leave it on, the lighter the sections become. You need to prepare the shirt meticulously to make sure the bleach doesn't bleed beyond the places you want it.
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 like the laundry room or garage. Wear old clothes you don't mind messing up, and cover or remove from the area anything you don't want bleached. 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 put the bleach into a spray bottle for more control. Have a drying area ready with an old towel to absorb excess bleach.
Rubber bands are the main tool needed when tie-dyeing; they block off certain areas of the fabric from the dye (or in this case, bleach) so that they don't change color. The ways you twist the fabric before securing it with the bands also determine the final design.
For a classic sunburst tie-dye design, lay the shirt flat. Pinch the center of it and pull upward about 1 to 2 inches; secure the pinch tightly with a rubber band. Pinch the same section another 1 to 2 inches down from the first rubber band and secure with another rubber band. Repeat until you've banded the whole shirt.
If you want a striped effect, fold the shirt together like an accordion length- or width-wise. Secure rubber bands around the length of the gathered fabric, about 1 to 2 inches apart each.
Once you've banded the shirt, dip it in a tub of bleach, or spray it with bleach for a more abstract pattern.
Let the shirt sit until it lightens to your desired shade. Once it does, roll it in the towel to soak up excess bleach.
Remove the rubber bands.
Set the shirt flat on a rack to dry.
After the bleach dries, wash the shirt to get rid of the caustic smell before you wear it.
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 up the shirt and wrap rubber bands around random areas.
Instead of using rubber bands, you can even lay shapes like plastic stars, cookie cutters or wooden square blocks over the fabric; spray the bleach on or splatter it over the shirt with an old paintbrush. Just be sure to use objects that can stand up to the bleach.
Experiment on scrap fabric to refine your technique. Lay it flat and spray bleach in a fine mist directly on it to create a splatter effect. This works especially well if you aim the spray upwards so it falls in an arc onto the fabric. You can also use an old paintbrush for thicker, more defined splatters, or use both techniques together.