Radio-waves, x-rays,radar and light are all electromagnetic radiation. What makes light visible and not the others is the sensitivity of our eyes. Radiation is defined by wavelength as seen on the chart below found on goggle images.
Infrared light is invisible to the human eye because it is not sensitive enough to pick up the long wavelengths, but as the wavelengths become slightly shorter deep reds become visible. At the other end of the visible spectrum violet has the shortest wavelength visible by the human eye, once the wavelengths become too short to be seen it is known as ultra violet. The visible wavelengths are the colours of the rainbow, on the chart above the two lines just after infrared and just before ultra violet are each end of the visible spectrum.
In the chart above, also found on google images, It shows how small the visible spectrum is in comparison to the range of wavelengths created by electromagnetic radiation. Light is the result of all these colours mixed together.
Sunlight contains all of the colours of the visible spectrum and some that are not visible, like infrared and ultra violet. Sunlight is our standard not only for brightness but also for colour. During the middle of the day it seems colourless, so we call it white light because we are so used to it and it is normal. Our eyes are more sensitive to some colours than others. If you look at the spectrum above, the brightest part is in the middle, yellow being the brightest. The colours at each end become darker, this is because our eyes have greater sensitivity to the middle of the spectrum and as you move further towards the invisible wavelengths the colours become darker than disappear.
Light becomes colour when some part of the spectrum is missing. Daylight normally becomes coloured in just two directions. These are red and blue. On a clear day when the sun becomes low in the sky there is more atmosphere between us and the sun. As the sun approaches the horizon it starts to become more yellow, then orange and eventually red at times. This is because the particles in the atmosphere, mainly nitrogen, scatter some of the light. As there is more atmosphere, and so particles, between us and the sun when it is low in the sky, more light is scattered. This scattering is selective and it is the shorter wavelengths that get scattered more easily, so violet and blue, leaving the longer ones more visible, orange and red.
The same scattering is what makes the sky look blue, the short wavelengths of blue and violet are spread out through the atmosphere and the longer wavelengths pass through easier. In the shade on a clear sunny day the light comes from the blue part of the sky, these wavelengths that have become scattered, causing a blue cast compared to the areas in full sunlight.
The possible colours of daylight are shown in the chart above, found on google images, with the relative degrees kelvin for each time of day. The colours go from red through orange and yellow to white, which is mid-day sun, then on to blue which is the colour of the sky. These colour temperatures are important in colour photography, as it normally desirable to use neutral light or compensate for the colour of the light. This is even more important when using tungsten artificial light.
The scale above, on which the colour of light is measured, is known as the colour temperature scale. Colour temperature depends on making a visual match between the colour of the light source and a standard. This standard is the colour that an inert material would glow when heated in a vacuum. The temperature for each colour is recorded and is known as degrees kelvin, or just K. This scale above is what all camera and film white balance is based on, with the aim to neutralise the colour of varying light.
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