In earlier studies, visual assessment rely on professional judgments. It may be too subjective and can't reflect public's opinions. This study uses color difference formula suggested by International Commission on Illumination to measure contrast between building and its background. The eye tracker is used to record fixation counts in area of interest and respondents were requested to answer questions about their perceived obviousness and preference. This study examines the relationships among fixation counts, obviousness and preference, and analyses the relationship between color contrast and the obviousness perceived. Result shows the higher fixation counts, the higher obviousness is perceived. The higher color contrast, the higher obviousness is perceived and the higher fixation counts.