What is a rainbow? It is the visible spectrum of light. In short, visible electromagnetic radiation, commonly known as VIBGYOR colours. And then there are the invisible rays such as X-rays, ultraviolet rays, infra-red rays, radio and microwave. We know how the invisible rays affect us, so it stands to reason that the visible spectrum also affects us. The good thing is the visible spectrum does not harm us, rather, the colours in the visible spectrum affect us in a positive way.
Colour therapy, also called chromotherapy, is a non-invasive therapy that uses the rainbow colours (visible spectrum) for healing. Colour was investigated as medicine since 2000 BC. The ancient Egyptians and Greeks used coloured minerals, stones, crystals, salves and dyes as remedies. The ancient Ayurvedic physician, Charaka, who lived in the sixth century BC, recommended sunlight to treat a variety of diseases. The ancient therapists may have been unaware of biological changes in the body due to colour treatment but they knew it was intrinsic to healing and restoring balance in the body . Here is how art based therapy helps in development of autistic children.
The principle of colour therapy
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Chakras, or the spinning wheel of light, are the energy centres in the human body. There are many chakras in the human body, but seven of them located mainly on the spinal cord are considered to be the major ones. From top to toe these are
Third eye chakra
Solar plexus chakra
These seven chakras resonate with the seven rainbow colours, that is, the chakras and their corresponding colours vibrate in the same frequencies. Healing therapies believe the disease is caused by the energy imbalance or some of the chakras (or maybe all the chakras) being blocked. The colour therapy is thus based on the principle that the qualities of each rainbow colour brings about a shift in the energy and restores energy balance in the body, thereby healing the body.
Physically speaking, the light through the eyes reaches not only the visual centres of the brain but also the hypothalamus that links the nervous system to the endocrine system and is involved in the key processes such as
Heart rate and blood pressure
Maintaining body temperature
Appetite and thirst
Electrolyte balance in the body
Stimulating pituitary glands to release hormones
What conditions can benefited by colour therapy?
In recent years, there has been much work on using the full spectrum and coloured light for mind-body healing.
Daniel Oren, from National Institutes of Mental Health, found that green light is more effective than red in treating seasonal affective disorder (SAD).
Coloured light has been found to be specifically beneficial in balancing the autonomic nervous system (ANS). ANS is crucial in most chronic and functional disorders as it regulates all of the automatic processes of the human body breathing, the beating of the heart, the functioning of the digestive tract and the stress response.
Laser light therapy has been shown to
Decrease healing time of wounds and ulcers
Facilitate bone remineralisation
Later in 1980s, a sophisticated system known as Esogetic Colorpuncture Therapy (ECT), for rebalancing the body s energy and treating specific disorders, was developed by Peter Mandel in Germany. He used a battery operated penlight with interchangeable coloured tips for focussing light on the points. Disorders that are successfully treated with this method are endocrine deficiency, inflammatory processes and degenerative diseases such as multiple sclerosis.
A review study in the American Journal of Acupuncture found that ECT was effective for treating a variety of difficult health problems like migraines, childhood insomnia, bronchitis, ADD or learning disorders, and uterine fibroids. 
Similarly, in Charles McWilliams system of chromopressure , colours of the spectrum are used on acupuncture and other pressure points. His ChromoPressure Unit is solar powered, concentrating sunlight over the points, and filtered through interchangeable coloured slides. The system can treat even obesity and chronic toxicity by stimulating lymphatic drainage. 
Colour applications for treating disorders
White light (full spectrum) later replaced by blue light as blue light was found to be more effective
Neonatal jaundice 
Bright white full spectrum light
Treatment of cancers, SAD (seasonal affective disorder, so-called winter depression), anorexia, bulimia nervosa, insomnia, jetlag, shift working, alcohol and drug dependency, and to reduce overall levels of medication. 
Treatment of rheumatoid arthritis. Exposure to blue light for variable periods of up to 15 min causes significant degree of pain relief. Treatment of a wide variety of psychological problems, including addictions, eating disorders and depression. Healing injured tissue and preventing scar tissue, as well as for burns and lung conditions. 
Effective in the treatment of cancer and constipation and in healing wounds.
Tranquilizing and calming effect within minutes of exposure. It suppresses hostile, aggressive and anxious behaviour.
Stimulates behaviour. Also, treats liver disease, gall bladder disease, and disorders of stomach and intestine.
As effective in treating seasonal affective disorder (SAD) as white light.  Also treats eye diseases and diabetes.
Treats of arteriosclerosis and cardiac insufficiency, and helps in controlling diabetes. 
Lemon yellow is the colour of the pancreas. It is a laxative and diuretic. It is a stimulant of brain, the liver and spleen. 
Light and colour therapy is not a magic solution to diseases but it is certainly an invaluable asset in the treatment of hormonal and mental disorders.
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1. Azeemi STY, Raza SM. A Critical Analysis of Chromotherapy and Its Scientific Evolution. Evidence-based Complementary and Alternative Medicine. 2005;2(4):481-488.
2. Croke M, Bourne RD. A review of recent research studies on the efficacy of EsogeticColorpuncture Therapy--A wholisticacu-light system.American Journal of Acupuncture. 1999;27(1-2):85-94.
3. Cocilovo, Anna. Colored Light Therapy: Overview Of Its History, Theory, Recent Developments And Clinical Applications Combined With Acupuncture. 1st ed. 1999.
4. Oren DA. Treatment of seasonal affective disorder with green light and red light. American Journal of Psychiatry. 1991 Apr;148(4):509-11.
5. Pandey A, Tripathi P, Pandey R, Srivatava R, Goswami S. Alternative therapies useful in the management of diabetes: A systematic review. Journal of Pharmacy &Bioallied Sciences. 2011;3(4):504-512.