Light is electromagnetic radiation of a certain wave length. The wave length determines the color of light. Short wave lengths are bright, blue and green colours, whereas long wave lengths represent warm yellow and red colors. Colors are often expressed as temperatures, where a low color temperature (2000-3000oK) represents red colors, while high color temperature (6000-7000oK) represents blue colors. White light is a combination of all colors, and day-light has a color temperature of about 6500oK)
Birds can observe light through their eyes, which is more or less similar to the way the human eyes see light. Especially short wavelengths (blue, green, bright white) are seen as very bright and of high intensity. These colors have a stimulating effect on behaviour and can increase activity, like eating. This is why green and blue colors are sometimes used for broilers. At the same time, they also can stimulate activitiy into unfavourable behaviour like pecking and cannibalism.
Birds respond to light also for reproduction. The light sensitive cells that stimulate the hormonal development for reproduction are located in the brain (under the skull). This is especially true with long wavelengths which can pass through the skull (comparable with sounds, where the long wavelengths from drums can penetrate through a wall or a car). Birds are therefore sensitive for red light for their reproduction.
As only the red part of the light is used for stimulation of reproduction, light sources must be examined for their wavelengths and colour spectrums to see how effective they are for reproduction.
Bright, blue lights of high intensity sometimes can be less stimulating for reproduction than warm red/yellow lights from low intensity, because only the red part of the light spectrum stimulates
When cold blue/white lights are used for breeders, higher intensities are needed to get the adequate level of red lights, but the blue/green part of the light will stimulate the activity of the birds, resulting into nervousness and pecking.
This is why red/yellow warm lights (low light temperature) are desirable for breeders, but blue/green cold lights (high light temperature) are often beneficial for broilers.