It’s easy to say that a crow is “all black.” You don’t have to be a birdwatcher to know that all of a crow’s feathers are black. But a closer look reveals that, at any given moment, even though all of the crow’s feathers are the same color, they do not all look the same. This is partly because of body contours and real-world lighting effects and partly because the feathers on different parts of the body have different textures.
Imagine a jacket made of alternating strips of different fabrics: velvet, cotton, and silk. Even if all of the fabric was dyed to exactly the same color (for example, black), the distinctive textures would make the strips of different fabrics obvious. The different textures of feathers on a bird create similar effects.