Visual designers should have a basic understanding of how color affects us. Color evokes emotional responses. It has sociological meanings. And when a single color is used repeatedly through a website to mean essentially the same thing, it aids usability. Users learn quickly what it means when they see an orange button again, or a green text link.
Users with color vision deficiencies (AKA color-blindness) may not be able to easily distinguish two colors based on hue alone. So make sure that your colors also contrast in terms of value (lightness versus darkness). If your website was being displayed in grayscale, would it remain usable?
A Guide to Color, UX, and Conversion Rates by Hannah Alvarez