Regarding the theme of color theory there are many articles, books, where is described the meaning of each color individually, and the rules of their interaction with each other. It's obvious that these basic characteristics are taken into account in web design, but they are used in complex with the above factors.
Because each color has its psychology, then the theme of web design will largely determine its color combination. For example, the predominance of black color talks about sophistication, style and elegance.
Blue — the color of safety, stability. It creates the atmosphere of confidence. For these qualities, it is used on medical, banking sites, for example.
If we talk about Dalmors.md (resource dedicated to fish production), the theme itself defined the color:
Green — symbolize purity, environmental friendliness, freshness. It is used frequently in the design of the sites that are specialized in organic products, food, flowers, and others:
Red — for some people it is perceived like the color of violence, and for others — the color of passion. But, not important how you regard this color, because in the end what remains true is the fact that: the red color effectively attracts attention, that is why, it is frequently used for online shops, not as a general background, but as a way to focus users attention. Namely, on these internet resources the main task is — to sell.
The visual part of a web page decides many things: how much time the users will be on the site, what things will attract their attention, what and how much they will buy, will they return for the second, third time. Also, the design closely intertwines with the technical part of the question: If you had a brilliantly designed main page and category pages, this won't bring any profit if the site is loading slowly or the links of red buttons lead to inappropriate pages.
The task of each business — to bring profit, which means you don't need to underestimate the importance of the visual part of the site. That is why we recommend ordering the web design development from professionals, who can provide a portfolio with successful works and a guarantee in the form of a contract. Otherwise, you risk getting an unpopular and unprofitable resource, even if it sells really high quality goods.