Style guides is visual guidelines for design process and to maintain identity across multiple images. I found the style guide quite useful especially when creating your own moldboard and make a colour scheme. The different categories:

Design Pillers - A core principles of what you are trying to depict. Genre, location, low tech/high tech, wealth/poor

Shape language - What are the primitive shapes. Rectangular, circular and triangular shapes.

Detail frequency - Low, medium or high frequency detail

Tech - Material details, ex stone buildings, wooden props, clean and sleek.

Colour/materials/decals - Colour scheme and workout primary, secondary and accent colours (highlight).