why sky is in light blue colour ?  

Go back to your physics book ( or google) and read again the part about refraction/ reflection and what is happening with the spectrum of its colors, when sunlight is crossing the Earth atmosphere. You'll remember... :)

Exactly don't know the answer for your question.
Taken from the fact that the sky is blue in its constancy i mean true blue. The sky is unwaveringly blue on a clear day but i don't know why no other color than blue for sky ..

Sky is blue because molecules in the air scatter blue light from the sun more than they scatter red light. When we look towards the sun at sunset, we see red and orange colours because the blue ight has been scattered out and away from the line of sight. This is all i found in goolgle

The Color Spectrum

In reality there are no real colors
just wave lengths

The longer the wavelength the
different the color appears

I believe it has to do with the moisture in the atmosphere, and with the sunlight reflecting off the surface of the ocean...

Hi Rajka...

It is really an intersting question...? I knew it when i was a kid, i read it in school but now i dont have a clue...I have read the responces you got but still they are not satisfactory...If you come up with a good answer, then please let me know....

Take care