Wrap an old towel around your shoulders, making sure the collar doesn't peek over the edge. Use a clip to secure the towel in place.
Use a wide-tooth comb to get all the tangles out of your hair. If you have very thick hair or want total coverage, divide your hair into three equal sections. Clip the top two out of the way.
Select a place that is well ventilated and features enough space that the spray won't coat the walls.
Put on protective gloves. Separate out a 1-inch strand from the bottom section, and hold it out from the head. Hold the spray bottle at least 10 inches from the strand. Using an even motion, spray the color from root to tip.
Use the color spray on all the strands in the bottom section. Release successive sections, and spray them in the same manner until all of the hair is covered.
Select a hair chalk that either matches or complements the original spray color, and use the chalk to fill in gaps left by the spray. Slide the chalk over the section of hair until the desired level of color saturation is achieved.
To prevent the temporary color from staining your clothes, spray a light-hold hairspray over all of your hair.