1. Economical Leather Dyes: Currently, the acid dyes and direct dyes available on the domestic market are primarily bright-colored mono-azo dyes; while they offer relatively poor leather dyeing fastness, they are low in cost. The products marketed as "Acid Black" and "Direct Black" fall into this category of economical leather dyes. 2. Acid Dyes: Acid dyes are anionic dyes used to color leather fibers in an acidic bath. Based on their dyeing properties and application methods, they are classified into strong acid dyes and weak acid dyes. Strong acid dyes feature simple structures, small organic molecules, and a high proportion of sulfonic acid groups. Examples include Weak Acid Orange GN, Weak Acid Brilliant Red B, Weak Acid Yellow 49, Weak Acid Black 210#, Weak Acid Black 242#, and Weak Acid Black 168#. Brown varieties include Weak Acid Brown 75#, Weak Acid Brown 348#, and Weak Acid Brown 349#. These dyes exist in an ionic state during the dyeing process and require a strongly acidic medium to achieve effective coloration, making them suitable for dyeing leather fibers. Properties of Acid Dyes: Acid dyes contain sulfonic acid groups (-SO3H), with a small number containing carboxylic acid groups (-COOH). Commercially, they are supplied as sodium salts, resulting in good solubility in water and alcohols. Acid dyes may undergo color or shade changes upon contact with metal ions such as calcium, magnesium, lead, copper, and iron. While washability and dry/wet rubbing fastness ratings are relatively low, the production process is simple and cost-effective. They offer vibrant colors, a complete color spectrum, and high dyeing fastness—typically achieving a lightfastness rating of 5–6. They are a primary product category in the high-end leather dye market, both domestically and internationally. 3. Direct Dyes: Direct dyes do not require a mordant when dyeing cotton, linen, or viscose cellulose fibers. Certain varieties of direct dyes can be applied directly to leather in an acidic medium. Direct dyes have high molecular weights and exhibit slow penetration rates. Compared to mono-azo acid dyes, these dyes involve a more complex production process yet command a low market price; furthermore, they perform poorly in terms of wet fastness, wash fastness, light fastness, and wet rub fastness on leather fibers, making them products that are being phased out of the leather dye market. 4. High-end, eco-friendly leather dyes: These offer advantages such as vibrant color, rapid penetration, and excellent resistance to light, washing, and migration. Free from aromatic amines and heavy metals, they are environmentally friendly and represent the mainstream products in both domestic and international leather dye markets.