Toned proof set.

1. What causes this type of toning? It's a "ghosting".
2. What can be done to prevent it?
3. Does it hurt the value, and if so, how much?

 

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1) It's haze, and very typical.
2) Store in a temperature and humidity controlled environment, but there's nothing to be done with those coins themselves except perhaps crack the case and dip them -- but then you've lost much of the value which is in the original mint packaging.
3) Yes, but hard to say and depends on whose doing the buying.

 

I personally wouldn't touch them...

 

Keep them as is. Eventually you might get some nice colors.

 

And, far more likely, eventually you'll get brown or black.