The Cooler The Colour - The Cooler Your Horse
Consider this: Would you be cooler wearing a light shirt or a dark shirt on a hot sunny day? If your science teacher or parents didn't convince you, the scientific fact is that white reflects the radiant energy rays of the sun and black absorbs them.
The same principle has a significant impact on the temperature of a horse. The darker the rug, the hotter the animal. Light coloured horse rugs reflect radiant energy and incorporate a cooling effect for your horse - so summer rugs should be a light colour to regulate your horses temperature.
Which colours reflect more light?
When light strikes a surface, some of its energy is reflected and some is absorbed. The color we perceive is an indication of the wavelength of light that is being reflected. White light contains all the wavelengths of the visible spectrum, so when the color white is being reflected, that means all of the wavelengths are being reflected and none of them absorbed, making white the most reflective color.
By using white or silver outer material, NASA can passively control 95-97% of the radiant heat.
Science made simple
If the color of a surface is anything other than white, it means that it absorbs light of some wavelengths. For example, a surface that appears red absorbs yellow, green, blue and violet light, while reflecting red light. A surface that appears green absorbs all colors except green. White light is a combination of all colors -- as is apparent when you shine a white light through a prism -- so anything that appears white reflects ALL wavelengths of light. Black is the least reflective color -- it's the color of a surface that absorbs all light.
Dark colours are generally best suited for cold and freezing conditions because they absorb more light than brighter than colours.
Tints and Shades
If a surface isn't white, then the closer its color is to white, the more light it reflects. Pastel and off-white colors reflect more light than deep tones. Adding white to a color is called tinting the color, and it increases the reflectivity. The contrasting procedure is to add black to decrease the reflectivity. This is called shading.
In 2012, researches from Hungary and Sweden found that on horses, light bounces off their dark coats and move along a horizontal plane. This light travels in waves that attract flies.
However, from a white coat, unpolarised light is reflected, which is less attractive to insects.