Computer- and web-based testing procedures are increasingly popular for the assessment of cognitive abilities and knowledge. This paper identified color red as a critical context factor that may influence the results. Two studies showed that color red may harm the performance in web-based tests of general knowledge. In Study 1 (N = 131) a red (vs. green) progress bar impeded the performance in a knowledge test, but only for the male participants. In Study 2 (N = 190) the color of the survey’s forward-button was manipulated (red vs. blue vs. mixed color) which led to a replication of the gender-dependent color effect. Evolutionary psychology and stereotype threat research explain why red impedes the activation of knowledge among men, but not among women.