A brand manual is a really great chance for a brand’s design team to explain the specific choices made for a brand. Check out this style guide for the channel Animal Planet particularly the page on colour palettes. Here it explains how each colour has been carefully sampled from the animal kingdom and then goes on to explain what emotions each colour conjure. When creating your own style guide, keep in mind that for whoever reads and executes your rules, it’ll be easy for them to just blindly follow design rules, but it’ll be even easier to follow them when each rule is explained and given reason. Check out the guidelines over here.