why is white gold haram as scientifically it is not a form of gold but a totally different metal from gold that is PLATINUM,just like iron or zinc,if a man can wear silver then why not platinum?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His Slave and Messenger.

It should not be said that white gold is unconditionally forbidden or permissible for men, but the issue should be detailed. Shaykh Muhammad Saalih Al-Munajjid detailed this issue well in his. We will quote what he mentioned in brief. He said:

After consulting experts, they said that the term “white gold” may be applied to a number of things:

1- It is applied to the metal platinum. It is permissible for men to wear it. The fact that people call it white gold does not make it forbidden, because it is just a name that is given to it.

2- The name white gold is given to the known yellow gold, but it is plated with a layer of platinum. Wearing this is forbidden for men, because wearing it means he is wearing yellow gold.

3- The name white gold is given to regular yellow gold that is mixed with a specific percentage of ”palladium” or another metal, of which more or less is added, depending on the standard of gold desired. This usage is very well known in stores selling gold.

It is well known that adding silver or copper to gold does not alter the fact that it is gold, and it is does not make it permissible to use it. That also applies to adding palladium to it.  Based on this, wearing white gold is forbidden for men, because in fact it is yellow gold but a substance has been added to it to change its color into white.

The Shaykh then quoted the Fatwa issued by the Permanent Committee in this regard. The Fatwa states that: “If gold is mixed with other metals, it still comes under the ruling that it is forbidden to differentiate in weight or measure when selling it for gold of the same type, and the exchange must be completed in one session, whether it is sold for gold of the same type or for silver or cash. It is forbidden for men to wear it, and it is forbidden to make vessels out of it. Calling it white gold does not change these rulings.”

It should be noted that what you mentioned that wearing silver is permissible for men is not a general ruling, since the silver that men are allowed to wear is only the ring, the sword's pommel and the like.

Allaah Knows best.