A pure platinum ring would never change the color of one's finger. Not only is it so prized because it is rare, but also because it is fairly inert and doesn't corrode. It's also fairly unlikely that a platinum alloy would corrode under most ring-wearing conditions.
In general, no. A green appearance would be indicative of a chemical reaction - another metal would be involved. So, pure platinum would not cause this to occur. Another metal alloyed with the platinum would be required. If green is observed, try to determine it's metallic composition. Before before purchasing a platinum ring, ask the seller about it's composition.
Pure platinum would not turn your finger green. However, both platinum and gold are alloyed when they are made into jewelry, for durability and value. One alloy metal used is copper, and another is nickel. If there is a lot of either metal in your ring, that is why it turned your finger green.
Rare metals such as Gold and Platinum do not leave anything green, however, if your ring is only plated and not pure Platinum. Then metal Nickel has been used as part of the plating process and you could get a green finger. Pure Gold or Platinum leave no green traces.
The only way that a ring would turn your finger green is if the ring was of a fake matter. Usually cheap jewelry and jewelry that wears easily will turn your fingers green.
No. I'm supposed to answer in 50 to 60 words, but that is rediculous as the question only warrants a single word answer. It is only low carat gold that does that cause of the copper and other minerals in the alloy. Silver will turn you black. Buy good gold or platinum and it won't happen.