In spatial design, designers aim to create environments that evoke specific emotions in users, in line with the designer's intentions—such as creating a comfortable or relaxing space. Among the various elements that influence users’ emotions in a designed space, color plays a particularly important role, as it is the fastest and most intuitively processed visual information and is known to readily evoke specific emotional responses. Therefore, it is crucial for designers to consider the emotions that colors in the spatial design may elicit to create environments that align with their emotional intentions. However, previous studies on spatial design have not sufficiently isolated and evaluated color alone in the emotional assessment of spaces. As a result, it has been difficult to determine the specific emotional effect of color itself within complex environments. To address this issue, the present study aims to isolate color from spatial design elements and clarify its emotional effect. To achieve this, we propose a method for extracting dominant colors that occupy a large proportion of the visual space—from designed environments. This method has the advantage of identifying the dominant colors used throughout the entire space, regardless of the shape or material to which the colors are applied. To objectively evaluate the emotional influence of differences in dominant colors, we employed a physiological approach using electroencephalography (EEG) and heart rate variability (HRV)methods that assess emotional responses independent of language. In the experiment, we used images of spatial designs composed of multiple colors based on predefined hypotheses. We extracted the dominant colors from these images and evaluated the emotional responses they elicited through physiological measures. Furthermore, to validate the emotional effects, we compared the similarity in dominant color characteristics with the similarity in emotional responses. The results suggest that even in environments composed of multiple colors, it may be possible to assess emotional effects by identifying the dominant colors in spatial design images.