personally I prefer the Dato. It has a more pleasing dial layout, more proportional compared to the 1815. And the big date works perfectly with the overall layout balance. Andy
Difference in temperature causes humid/moist in safes. My guess is a slight humidity can't cause the blue to vanish, unless the temperature difference from low to high is tremendous, then it may do something rare. I really hope you will get a good repair/replacement disc for this amazing timepiece.