Consider the following scenario:Emile is creating an app that lets people rent bicycles. Users can check-in, check out, and pay for their rental with it. Emile wants users to associate the brand color—vermillion red—with the check-out process by indicating successful progress with checkmarks in this color. Emile wants to make buttons, such as “Next” and “Complete Order” to follow the same theme. Which Gestalt principle is this comparable to?

Gestalt Principles are principles/laws of human perception that describe how humans group similar elements, recognize patterns and simplify complex images when we perceive objects.
Designers use the principles to organize content on websites and other interfaces so it is aesthetically pleasing and easy to understand.