The purpose of this study was to verify the moderating effects of mature and immature defense mechanisms in the relationship between color preference and optimism in college students. For the study, data was collected from 414 Chungcheong-do A college students. A research model was set up so that color preference and ego-defense mechanism affeced optimism, while ego-defense mechanism acted as a moderating variable for color preference and optimism. Hierarchical regression analysis was performed using the SPSS 23.0 program. The results of study showed that, first, color preference, optimism, and mature defense mechanisms were positively correlated. Second, the moderating effect of mature defense mechanisms was confirmed in the relationship between color preference and optimism. Third, in the relationship between color preference and optimism, immature defense mechanisms did not show a moderating effect. This study is significant because it verifies the role of a mature defense mechanism as a moderating variable in the relationship between color preference and optimism in college students. Based on these results, the implications of this study and suggestions for further research were presented.