This study investigates the impact of mixed-reality (MR) interface background colors on cognitive performance and subjective workload in a simulated space emergency scenario. Thirty-five participants performed a modified Space Stroop task under five different MR background colors: red, green, blue, yellow, and white, with response times, task accuracy, and perceived cognitive load assessed. The results revealed significant differences in response times, with participants demonstrating the best performance under the yellow background, exhibiting faster response times and higher accuracy compared to the other colors. While no significant correlation was found between response times and accuracy, yellow backgrounds appeared to offer a cognitive advantage, likely due to improved visibility and reduced cognitive load. Subjective workload assessments using the NASA Task Load Index (NASA-TLX) indicated that participants experienced lower cognitive demand and frustration with the yellow interface. These findings suggest that yellow MR backgrounds may optimize performance and emotional stability during high-stress tasks, such as emergency repairs in space. Further analysis revealed no significant learning or fatigue effects, strengthening the conclusion that background color was the primary driver of performance differences. The study highlights the potential for yellow MR backgrounds to enhance astronaut efficiency and decision-making during space missions, providing a foundation for future interface design recommendations. Future research should explore individual differences in color perception and test these results in real-world space environments to further validate the findings.