Three studies of the psychological and physiological effects on people of colored room interiors are described. Experiment 1 compared a colorful and a gray room, whereas in experiments 2 and 3 red and blue rooms were compared. The results indicate that the color of an interior space will have effects on many different levels. The perception of the room itself was affected, and the colors also had an impact on the emotions and physiology of those who staved in the rooms. Strong, especially red. colors and patterns put the brain into a more excited state. sometimes to such an extent as to cause a paradoxical slowing of the heart rate. Introvert persons. as well as those already in a negative mood, became more affected than others, which caused severe changes in their performance. The series of experiments described here were among the first to be carried out in full-scale rooms painted or otherwise decorated in various colors. One practical implication is that a moderate use of good color design will serve to improve the overall mood and well-being of people. In future research more emphasis should be placed on color, as just one component in the highly complex real-life situations. (c) 2009 Wiley Periodicals, Inc. Col Res Appl, 34, 141-151 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.20476