It's possible to vary in multiple channels, i.e. color, shape, angle, size, etc... AND there are multiple degrees of difference within each channel. So if you have more than a few items in a design, there are so many different possible combinations. For example, some items could only differ in size(in a particular difference in size), while some items can be different in color and shape(in particular difference of hue/sat/value and angles).
While design books and courses give you vague principles and leave it up to a designer's intuition, I figured there must be some more systematized and detailed guidelines based on recent cognitive and neuroscience research.