Hair dye can change the color of hair through temporary, semi-permanent, demi-permanent, or permanent methods. Temporary dyes coat the hair shaft but wash out easily. Semi-permanent dyes penetrate slightly and last a few washes. Demi-permanent dyes use low-level peroxide to deposit dye inside the hair without lifting color. Permanent dyes use higher peroxide levels to both lift and deposit dye, permanently changing hair color and structure. Common ingredients like paraphenylenediamine (PPD) can cause allergic reactions or toxicity if misused or ingested.