Compositions of color should appear to have depth to them. This is called value and is also characterized by the amount of black an area of a composition has. Value plays a large role in “false-color” images where the value is correct but the hue is not.

Different values are almost always present in representation. without them an image will often appear flat, or appear to have no depth.

Images can have incorrect or false colors, color that we would not see or represent light not in the visible spectrum, or are simply exchanged for colors found in nature. For example, a brown dog is bright orange and with the correct depth of value we would still see the intended image. For example with vegetated assemblies: all of the plants have different values and sizes create depth, where smaller and darker plants recede while lighter and larger plants come to the foreground.

Depending on how you want the wall to appear, plants can be chosen to: make a wall visually and literally have more depth though different hues and shading patterns through the characteristics of the plants; or appear flat with different expressions of similar criteria but choosing only one species or a finer or shallower plant species.

Your eye perceives depth though the differences or hues and in the shadow found at the edges of different adjacent plants. I would look at Patrick Blanc, or the book Facade Greenery by van Uffelen for examples.