Color psychology is a branch of psychology that investigates the effects of color on the behavior and feelings of human beings. It is a very young discipline, still in the process of performing the most fundamental empirical studies. This means that any theories coming out of it are necessarily tenuous. Thus any therapies based upon it are typically seen by mainstream psychologists as falling into the category of alternative medicine.
One of the key points to understand is that color psychology is different from color symbolism. The importance and effects of colors according to color psychology are natural physiological responses uncovered by empirical study. In color symbolism, on the other hand, the importance and effects of colors are revealed by cultural context and may differ across cultures and across styles of art. So for example, in color symbolism red is a high impact color that is typically associated with danger but in color psychology the colors of danger have been found to be yellow and black.
The empirical evidence provided by color psychology studies is very varied and can produce some surprising and seemingly obscure results. The color red has been found to increase respiration rate and heighten blood pressure, for example. It has also been found that there are typically more positive connotations for colors than negative. Black, though, has negative perception in many cultures, even to the point where referees are more likely to penalize sports teams wearing black shirts.
But ultimately it would seem that culturally developed color meanings can more often over-ride what would be the natural responses to colors. This is a particularly potent force when the colors are associated with political ideologies, such as red for Socialism and brown for Nazism, or with religions, such as green for Catholicism and orange for Protestantism. The bottom line is that there is no universal psychological reaction to any particular color.