Orange.
I know it's becoming a land-slide, but that's my call. Unless you're a gym coach or life guard, odds are pretty high that the whistle is for emergency signaling. In that case, you WANT to be found, so even a fleck of safety orange in a field of vision might help. In addition, as has been pointed out already, it will be easier to find if dropped (for instance, in leaf clutter, where both green and black could easily disappear). As orange is a pretty darn rare color in nature, it tends to stand out no matter where you go.
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