The affinity of dyes depends on chemical and physical structure of the protein fibers, the character and distribution of the tanning agents, fatliquors and other reagents that precede or follow coloring. Cross-coloring is essential on suedes and uniform penetration is important on suedes which are buffed after coloring. Anionic or cationic synthetic dyestuffs are usually used. Available methods are 1) selection of dyes that have good inherent penetration and 2) alteration of the leather surface to reduce its affinity for the dyes.