Leather dyes are pigments mixed with a base (usually alcohol, oil, or water), and applied to leather to change its color. Each types has its own performance characteristics, so use would depend on personal preference and the specific project on which they’re being used.
For best results when dyeing leather, a crafter can usually always dye darker, though not lighter. For example, one can dye a vegetable tanned belt gray. Then brown. Then black. However, they can’t nearly as easily dye a black leather belt a lighter color such as gray, or white.
In general, dyeing leather requires proper ventilation to ensure a safe working environment. Also, leather dyes are temperature sensitive, and should be used within the temperature range recommended on any specific dye that you’re using.