Fish leather is made by tanning fish skin. Once the scales are removed, the skin retains a pattern of small pockets where they were anchored. This pattern is the signature of the material and the main reason it is chosen for boots.
Different species produce very different looks. A small-scaled fish yields a fine, dense texture; a large-scaled fish such as Pirarucu yields a bold, open pattern. Finishing techniques — glazing, matte treatments, sanding and dyeing — further change how light moves across the surface.
When comparing boots, it helps to look at the leather in natural light and from a few angles. The same color can appear quite different on a fine pattern and a bold one.