Start with a freshly laundered or cleaned coat to ensure even coverage of dye. If your coat is stained or dirty, you will only dye your stains and dirt a darker color.
Read your coat's label. Dye works best on 100 percent cotton, linen, wool, silk or ramie, and on synthetics like rayon, nylon or acetate. If your trench coat is a fiber blend with at least 60 percent cotton, the dye will tint the coat but you will not get rich vibrant color. Does your coat say "dry clean" or "washable in cold water" only? If yes, ditch the dye idea. Your coat will fit your 2-year-old niece after you dye it.
Do a swatch test. Mix a small amount dye solution in hot water as per the instructions on the box. Test a swatch of your coat, like maybe the tip of the belt, and let it dry. If your coat is in good shape with no stains, the swatch, when dry, is what your coat will look like.
Mix your solution. Use enough dye for the size of the coat. For larger, darker items, choose the liquid formula over the powder, which is more concentrated. For best results, use several packages of dye. The color will be richer and darker if the coat accepts dye well.
Stir dye into a tub of hot water or the washing machine until it's well-mixed. Do not pour the dye on the clothing or your coat will be spotted. Darker, deeper colors use a temperature of 140 degrees.
Rinse the clean trench coat in clean, clear water to wet it thoroughly before you add it to the dye bath. This will create fewer spots or unevenness.
Put your wet trench coat in the water. Stir or agitate constantly to keep the color even, or place it in the washing machine and let it do the agitation. The longer the trench is immersed, the deeper the color; you can keep it immersed for up to one hour as long as the water stays hot.
Remove the coat from the dye when it's done. Rinse in cool water and wash in cold. Note that your coat will bleed every time you wash it from now on, and you should keep it separate from other articles of clothing if you do wash it.
Clean up your washing machine or sink with 1/2 cup bleach in hot water.
For best results, take your trench to a tailor or dry cleaner or someone who specializes in dying fabric. Dyeing the coat at home can be tricky.
If your coat is 50 percent or more polyester or 100 percent acrylic, don't bother trying to dye the coat. It won't take.
If your lining is acetate, it will not dye. Remove it if all possible before dying the coat, then replace it after the coat has been dyed.