why ice is sometimes white & some times transparent

There is a Henry's law in chemistry.
"At a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid."
in simple way
"Gas solubility in water increase as the temperature decrease."
whiteness of ice is mainly dependant on two things 1) Method of ice making 2) what kind of water you are using for ice making i.e.Properties of water
when we put water in ice can it might have dissoved some gases from air or air itself get dissolved in water.
when you start ice making process, temperature of water drops and as its temperature decreases it dissoves more gases/air in it.
these bubbles of gases get enclosed inside ice help to give ice white appearance.
when actual ice formation starts ,first ice formed near to walls of ice can,at the same time these gas bubbles moves to centre part where water is presents,that is why you find ice is more white at centre and more transparent or clear at surfaces.
you can make clear ice by using boiled (deaerated) water.