Un-dyed leather means, nude leather, with nothing covered up - so they will have some natural range marks, scars, and other blemishes. In other words, they are not processed to make them appear perfect, like something you would get in a department store. When you get your brand new un-dyed vegetable-tanned leather good, it looks a natural color. The hide is effectively naked and so will age and darken with the sun just like your own skin. Aging will develop a rich color and patina. Sun exposure, daily handling, water, and conditioning give natural vegetable tanned leather a rustic look over time.  Read more about vegetable-tanned leather.