Within the context of human-computer interaction, this thesis investigates the effects of various design elements within newsletters on user interaction and engagement. The aim of using A/B testing was to study how different design elements including color, text in the call-to-action button and its position impact user engagement. Through the implementation of A/B testing on a sample of subscribers, the research aims to identify which combinations of visual and textual elements result in higher open rates of the newsletters, clicks on the CTA button, and generally greater user engagement. The purpose of the conducted A/B testing was to enable a deeper understanding of user preferences and reactions to weekly newsletters of a specific company, facilitating decisionmaking regarding content design. Considering the accelerated development of technology and changes in user behavior, email marketing has become an essential tool for organizations seeking to establish and maintain contact with users. In this context, A/B testing stands out as an effective approach for precisely measuring the effects of different elements within these campaigns. This work explores strategies, techniques, and challenges related to A/B testing in the context of human-computer interaction, along with theoretical foundations regarding the role of colors. The thesis also analyzes the effects of colors on user reactions during the use of newsletters. By properly applying these tools, organizations can enhance their campaigns, increase user engagement, and ensure long-term growth and success in the market. In the research section of the paper, three A/B tests were conducted through different versions of the newsletter. The results showed that various design elements influence user interaction and engagement. While the color and text of the CTA button did not have a significant effect, the position of the CTA button played an important role in increasing engagement. Throughout all three analyses, the impact of design decisions on user engagement was highlighted, emphasizing the importance of human-computer interaction principles in developing effective digital communication tools. A design based on these principles is crucial for implementing successful digital marketing campaigns. Regular testing through A/B testing is essential for understanding the elements that influence user behavior, enabling quicker adjustments to strategies and ensuring positive results in campaigns.