Guides are helpful because they allow us to benefit from someone else's trial and error. They also show us where our personal preference and instinct might have otherwise led us astray.
Of course, we should test these theories for ourselves to see if they hold water. For what it's worth, I have experimented with color combinations based upon this theory, and have found it to hold true.
Certain shades of grey, brown, yellow and blue (especially) can also go with khaki but not all of them - hence my reference to 'minefield'.On the contrary, all colors go with khaki: "Neutrals work well with all colors or may be worn together." http://www.askandyaboutclothes.com/C...lorReality.htm That is why neutrals are called "neutrals."
The designer Alan Flusser's large-format Dressing the Man is also an excellent guide, because it contains illustrations following these rules and breaking these rules, and explanations why they work or don't work, respectively. It is a pricey book, but worth a look at least. If one thought that color theory was complicated, check out his examples of matching four stripes and patterns harmoniously.