It depends on what kind of color therapy treatments your friend is receiving because some are legitimate and others are entirely New Age and/or just plain bogus.
In the New Age version, color therapists, aka chromotherapists claim that they can use light in the form of color to balance energy in a person's body, mind or spirit. There are a plethora of these practitioners on the web, many of whom have adopted the ayurvedic medicine approach which asserts that each of the seven chakras (energy centers) of the body correspond to a particular body organ or function, but also to a specific color. Disease or illness occurs when the chakras become imbalanced, a situation that can be corrected by applying the appropriate color.
An equally bogus branch of color therapy appears to have been founded by a man named Dinshah P. Ghadiali (1873-1966). A child prodigy (at least by his own assertion), he supposedly began studying medicine at the age of 14 and became interested in color healing after curing a girl who was dying of colitis by exposing her to light from a kerosene lamp that was fitted with an indigo-colored filter. He also gave her milk that was placed in a bottle of the same color and exposed to sunlight. Within three days, the girl was well. This convinced Dinshah that he was on to something and he founded the Ectro-Medical Hall in Surat, India. By the time he emigrated to the U.S. in 1911, he had developed a theory about his colored light treatment.
As this article explains, Dinshah believed that all of the elements exhibits a preponderance of one of the seven prismatic colors. "Oxygen, hydrogen, nitrogen, and carbon, the elements that make up 97% of the body, are associated with blue, red, green, and yellow. In a healthy person these colors are balanced, but they fall out of balance when disease strikes. The therapy is simple: to cure a disease, administer the colors that are lacking or reduce the colors that have become too brilliant."
He created a Spectro-Chrome to treat patients with colored light. It was a box outfitted with a 1000-watt light bulb in it and an opening that was fitted with colored filters. The five filters could be deployed singly or in pairs to produce twelve different colors. He developed a therapeutic system detailing which color to use for which illness - such as using green to treat the pituitary and red to improve sexual vitality.
Dinshah ran into trouble with both the scientific and the legal establishment in the U.S. - the former because of his advice to patients to forgo conventional medicine for his cures instead, and the latter because of sexual and other improprieties that landed him in jail for four years.
Needless to say, there is no scientific evidence to support any of the above claims.
However, the scientific version of color therapy has been in use for a long time. According to the American Cancer Society, physicians prescribe light boxes that mimic sunlight to treat people suffering from seasonal affective disorders (SAD). Ultraviolet light is used to treat psoriasis and cutaneous T-cel lymphoma. Photodynamic therapy has been found to be helpful in treating certain cancers or precancers of the skin, esophagus, and lungs, and is now being tested against other types of cancer. Phototherapy lights are also used to treat premature infants who are suffering from jaundice and other troubles.
This article gives more background information on color therapy which should help you ask your friend the right kind of questions to determine what kind of treatment she's using - the bogus kind or a legitimate form.