In my final year of Interactive Arts and Science at Brock University, I wrote my undergraduate thesis. In order to understand the effects that a website’s colours have on its viewers and marketing effectiveness, this thesis examines several different aspects of colour theory including colour perception, psychology, marketing affordances, and accessibility. It then takes these concepts and applies them to the Amazon, Rakuten, and Craigslist websites to analyze how their use of colour may be affecting their website’s marketing effectiveness in both positive and negative ways. As I demonstrate, colour is most effective when it adopts the biological, psychological, cultural, and social affordances of its viewers. Using the examined theories and examples, it then provides suggestions on how to use colour in UX/UI design in the most efficient ways possible.