if they live on Mars then they are blue, but they only look purple because the red dust in the atmosphere changes the reflected color. :-)

"Prove me wrong!"

Unfortunately there is not enough information on the subject of people that "turn green only after they die" and "In life they're purple." Therefore, neither you nor I can "prove" that they do or do not exist.

Now, if there were hundreds of people that had seen them and could describe their behavior, also if there were photographs and movies depicting their features, then the weight of evidence would be greater for their existence than for their nonexistence.
Of course there would be people who would argue that the pictures are fake and the people reporting such things are crazy. The "truth" of the matter would remain in obscurity until the "evidence" became overwhelming one way or another.

What type and what quantity of "evidence" will convince people of the existence of purple people, UFOs, or water and life on Mars, is anybody's guess. Moving big blocks of stone with sound waves, and constructing stereo vision for autonomous robots would be much easier and cheaper than proving that life, either exists or does not exist, on Mars.

Come to think of it I have not seen a real "platypus" can anyone "prove" that the pictures that I have seen are not fake, or that the people that say they exist are not "making it all up"? Why do I and everyone else "believe" that the platypus is real? How much "evidence" did it take to convince people of the reality of the "platypus"?

Truth
The truth isn't always beauty, but the hunger for it is.