Blue Sky, Red Sunset, White Clouds

Not all colors are the result of subtraction and addition of light. Some colors like the blue sky result from the selective scattering of the colors of light by particles in the atmosphere. The color scattered by the particles depends on the size of the particles themselves.

BLUE SKY
Violet is scattered more than blue; our eyes are not very sensitive to violet. The scattered blue light predominates, so we see the sky as blue. The Color of the sky varies with the conditions of the atmosphere. On days when the atmosphere contains water vapor, particles of dust, and others bigger than nitrogen and oxygen, the sky appears less blue compared to clear dry days. Why do we have Blue Sky?

The blue color of the sky is due to Rayleigh scattering. As light moves through the atmosphere, most of the longer wavelengths pass straight through. Little of the red, orange and yellow light is affected by the air. However, much of the shorter wavelength light is absorbed by the gas molecules. The absorbed blue light is then radiated in different directions. It gets scattered all around the sky. Whichever direction you look, some of this scattered blue light reaches you. Since you see the blue light from everywhere overhead, the sky looks blue. RED SUNSET

The sky around the setting sun may take on many colors. The most spectacular shows occur when the air contains many small particles of dust or water. These particles reflect light in all directions. Then, as some of the light heads towards you, different amounts of the shorter wavelength colors are scattered out. You see the longer wavelengths, and the sky appears red, pink or orange. Why is the Sunset Red?

As the sun begins to set, the light must travel farther through the atmosphere before it gets to you. More of the light is reflected and scattered. As less reaches you directly, the sun appears less bright. The color of the sun itself appears to change, first to orange and then to red. This is because even more of the short wavelength blues and greens are now scattered. Only the longer wavelengths are left in the direct beam that reaches your eyes. WHITE CLOUDS

The whiteness of the clouds is due to light of all colors being dispersed by the water/ice of the clouds. Clouds quite a distance away appear yellow due to the blue wavelengths being dispersed more than the yellow and red wavelengths by the air and particles in the air (the ‘clear’ sky is blue!) Not all clouds are pure white; however, for those that are white, it is due to their altitude and the reflection of sunlight. For example, a cirrus cloud has an altitude of approximately 8km above sea level. At such extreme altitudes all high-level clouds are made up only of ice crystals, as the water vapor from which they are initially formed has frozen. Why is the Clouds White?

The ice crystals reflect sunlight. When flying above clouds during the day, they are always bright white. When we get dark clouds, they are so thick that they soak up most of the sunlight or reflect it upwards, and so things aren’t as bright below. Storm clouds are the thickest clouds, and look the darkest from down below, though they still look bright white if we see them from above. Gray color of the clouds is caused by higher clouds casting their shadow on lower-based clouds, or that the clouds are so dense that their top parts absorb most of the sunlight, casting their own shadow along their base, making them dark on the bottom.

Conclusion
Therefore I conclude that not all are the result of subtraction and addition of light. Some colors like the blue sky result from selective scattering of the colors of light by particles in the atmosphere. The color scattered by the particles depends on the size of the particles themselves.

References

http://answers.wikia.com/wiki/Blue_sky_red_sunset_and_white_clouds http://www.sciencemadesimple.com/sky_blue.html
http://wiki.answers.com/Q/Why_are_clouds_white
Science and Technology Physics textbook