Why Hindu Gods were blue-skinned?

Not all the gods in Hinduism were blue-skinned, in fact, only Ram, Vishnu, Krishna - and Krishna was the reincarnation of Vishnu .. and Shiva .. We know that Lord Shiva was not born blue - but became so as a result of poisoning .. This is important, now I will explain .. Nowadays we use silver in water-filters because of antibacterial properties of silver ions. This is not a secret knowledge some people use it for medicinal purposes - for example, in the USA you can buy a colloidal solution of silver. This is a real photo of a person who overdose colloidal solution of silver for too long - this effect is permanent. How in ancient India it was possible to obtain a concentrated solution of silver ions without electricity? For example, if you add silver powder to a jug - this will increase the contact area of ​​water and metal hundreds of thousands of times .. and it will get anibacterial proerties.. but if you overdose it, you know what going to happen...