Shader Complexity Mode is used to visualize the number of shader instructions being used to calculate each pixel of your scene. It is generally a good indication of how performance-friendly your scene will be. In general, it is used to test overall performance for your base scene, as well as to optimize particle effects, which tend to cause performance spikes with a large amount of overdraw for a short period of time.
Only instruction count is used to calculate shader complexity, which may not always be accurate. For example, a shader with 16 instructions, all texture lookups, will be much slower on all platforms than a shader with 16 math instructions. Also shaders which contain loops that are not unrolled will not be represented accurately by the instruction count, this is mainly an issue for vertex shaders. Overall the instruction count is a good metric in the vast majority of cases.
The view mode uses a color spectrum to indicate how expensive the scene is. Green through red represent a linear relationship of "very inexpensive" to "expensive", while pink and white indicate a large jump to "very expensive" pixels. Small areas of white can be tolerated, but if the majority of your screen is covered in bright red or white, the performance will be poor.
Shader Complexity Coloration