Translation =/= Equivalent
It would be like translating "Satoshi" to "Wisdom", "Kasumi" to "Fog". Really, it is a NAME. Translating the names literally is just wrong. If there was a word "purachina" somewhere in the text, used as "platinum" (the element), then it would translate as "Platinum" to English. But in this case, it's a name.
"Red", "Gold", "Ruby" etc. are a different case because in their cases their names were actually borrowed from English directly, so "Reddo" and "Red" are actually the same word, direct equivalent. However, this is not the case of "Purachina", which is not a direct equivalent of English "Platinum", but rather a direct equivalent of Portuguese "Platina".
It's really frustrating when people try to make English words out of Japanese words. They always get some peculiar-sounding Engrish. This time, we get a word borrowings from Portuguese, so if one actually wants to use an alternate romaji, the only possible choice is "Platina". "Platinum" is not a Romaji, it's a translation.
You just have to understand that not anything in Japanese can be Englished. And that there is a difference between Romaji and Translation.