The discussion of color in HCI often remains restricted to issues of legibility, aesthetics or color preferences. Little attention has been given to the emotional and semantic effects of color on digital content. At the example of black and white, this paper reviews previous studies in psychology and reports an experiment that investigates the influence of black, white and gray user interface backgrounds on the perception of sentiment in chat messages on a social media platform (Twitch.tv). Of sixty-seven participants, those who rated the messages against a black background perceived them more negatively than those who worked against a white background. The results suggest that user sentiment perception can be influenced by interface color, especially for ambiguous textual content laced with irony and sarcasm. We claim that this knowledge can be applied in persuasive interaction and user experience design across the entirety of the digital landscape.