Why do clouds appear only and only red at night

Question

I have till now in my life I have always seen clouds only white or black or grey in day but in night why they are always red I could never found it's reason. And also sometimes in day time I see patches in sky like colourful as if any oil stain is there in sky.

Answer 1

Clouds are the color of whatever wavelength of light strikes them. Before sunrise and after sunset, when the sky darkens but light comes from beyond the horizon, sunlight passes through a greater thickness of atmosphere than when it is shining straight down from above.

When the Sun is directly overhead, Rayleigh scattering sorts out short-wavelength ultraviolet, violet, and blue light. That's why the sky seems blue during the day: all the blue wavelengths, which are about the same length as the molecules in the air, illuminate the molecules of atmosphere. The longer wavelengths are subject to Mie scattering from water vapor in the air.

But just before sunrise and just after sunset, light rays from the Sun follow a longer path through the densest part of the atmosphere near the bend in the Earth's surface at the horizon. They undergo intensive Rayleigh and Mie scattering, which filters out most of the blue light, and leaves only the longer wavelengths, first yellow and then red. That's why as the Earth rolls away from the Sun, presenting a thicker denser atmosphere for light to penetrate, the color of the clouds changes from yellow to orange to red.

Here is a better explanation with illustrations, from the National Weather Service: http://www.srh.noaa.gov/jetstream/clouds/color.htm.