Fabric dye is a great product to use when refashioning clothing for costumes. You can use it to take the brightness out of a color, add richness, or add additional color. Rit Dye has helpful info on FAQs when dyeing such as what color formulas to use, how to achieve a new color over an existing color, and what fabrics dye best.
Take a look at this Rit Dye Project Page for great ideas on refashioning with color!