Why is sky blue?

The sky is blue due to a phenomenon called Raleigh scattering. This scattering refers to the scattering of electromagnetic radiation (of which light is a form) by particles of a much smaller wavelength. These shorter wavelengths correspond to blue hues, hence why when we look at the sky, we see it as blue.

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Regarding this, what is the observation of the clouds are dark?

The base or bottom of the cloud is usually darker because sunlight must pass through billions of cloud water droplets or ice crystals from the cloud top or sides (depending on the suns position in the sky). What are the examples of inference? Inference is using observation and background to reach a logical conclusion. You probably practice inference every day. For example, if you see someone eating a new food and he or she makes a face, then you infer he does not like it. Or if someone slams a door, you can infer that she is upset about something.

In respect to this, what is the difference between observation vs inference?

An observation uses your five senses, while an inference is a conclusion we draw based on our observations. It might be helpful to have some examples. Observations can be made only with the five senses. Inferences involve a decision being made about something you observe. Why cloud is black and white? It's pretty well-known that most clouds are white, while rain clouds are usually a darker shade of gray. That is, a cloud gets thicker and denser as it gathers more water droplets and ice crystals - the thicker it gets, the more light it scatters, resulting in less light penetrating all the way through it.

Thereof, why do clouds change color?

The color of a cloud depends primarily upon the color of the light it receives. The Earth's natural source of light is the sun which provides 'white' light. The colors change as the wavelength increases from violet to indigo to blue, green, yellow, orange, red and deep red.