Summarize:

There are many popular brands and hundreds of different colors to choose from. Semi-permanent dyes can be found in a variety of places, ranging from grocery stores to specialized fashion shops, with the biggest selection at beauty supply stores. Pick the color that's your favorite, matches your wardrobe best, and/or is most flattering for your eye color and skin tone. Semi-permanent dyes work on all hair colors, but show up brightest on light hair. On very dark hair, though, they will appear as a very subtle sheen. Bleaching hair will make the color turn out brighter on all hair types. If you want intense, bright colors and you have darker hair, you will need to bleach it. Just keep in mind that bleaching may damage your hair. A patch test is an easy way to check for allergens at home. This step is very important, especially if you have sensitive skin. If you go straight to dyeing your whole head without checking, you may end up with an itchy scalp or permanent hair loss. Dab a small amount of dye onto a cotton adhesive pad and place it somewhere on your skin that isn't too sensitive or typically exposed like your upper back. Leave the patch on for however long you plan on leaving the dye in your hair (usually an hour). Remove the patch and rinse the dye from your skin. Wait another 48 hours before dyeing your hair. Do not use this dye if your skin experiences itching, redness, or swelling. If your skin starts itching while the patch is still on, remove it immediately and scrub away the dye with soap and water. Select a small strand of hair that is not on the surface of your hairdo. Dye this strand following the instructions on the bottle. This step can be skipped if you don't care about the exact shade of your new hair color. If you've never bleached your hair before, you may want to do a strand test with both your bleaching method and then with the dye.
Find the color you want. Bleach your hair if necessary. Perform a patch test. Do a strand test.