Will gold plated jewelry leave a green mark on your skin?

That depends what the gold is alloyed with and what is under the gold. If the gold is alloyed with a cheap material or is covering a cheap material (usually copper) then it might turn your skin green when it oxidizes (copper turns green when it oxidizes - like that Statue of Liberty). If it's gold over silver (and you trust the jeweler), then you should be fine.