Why does using only one color with different shades (like all blues) make you really notice how bumpy or smooth things are in a photo?

Using only one color with different shades, known as a monochromatic palette, makes the bumpiness or smoothness of objects in a photo more noticeable because it eliminates hue (the pure color itself, like red or blue) as a primary visual cue. Our visual system then heavily relies on value, also called luminance, which is the lightness or darkness of that single color. The perception of three-dimensional form, including texture, is fundamentally based on how light interacts with a surface, creating highlights (bright areas) and shadows (dark areas). On a bumpy or textured surface, light hits different parts at varying angles, causing rapid and distinct changes in value. Raised areas catch more light, appearing brighter, while recessed areas or areas angled away from the light fall into shadow, appearing darker. These abrupt shifts in value, or value contrast (the difference between light and dark), across short distances clearly define each individual bump and indentation, emphasizing the unevenness. Conversely, on a smooth surface, light reflects or diffuses more uniformly, resulting in gradual, continuous, and often subtle transitions in value across the surface. The lack of sharp value changes indicates a uniform or gently curving form, making its smoothness evident. By removing the interpretative complexity of multiple colors, our brain focuses intensely on these value variations. This heightened sensitivity to the interplay of light and shadow, which directly reveals surface geometry, makes even subtle differences in depth and form significantly more apparent and easier for our visual system to process and understand.