The effects of background colors on the scores of a computer-based English test were analyzed. Eight combinations of black text and a background color were chosen to examine if test takers' scores differed, depending on the characteristics of a background color. Two hundred and forty six Japanese college students participated in the experiment. A computer-based test (CBT) that resembles the TOEIC® Test was used and the subjects, divided into eight groups, answered the same questions displayed on a background of eight different colors. The results showed that the subjects who took the test with light blue and blue background scored the highest and the second highest. Although many CBTs use the combination of a white background and black text, this combination was ranked the second worst. The results indicate that colors with higher values both in luminance and in brightness can cause lower performance, and test taker's color preference does not affect his performance.