Why do my egg shells have discoloration?
You didn't post pictures, so I'm going out on a limb here:

Looking at some eggs out of my fridge, I see grayish spots on some white eggs. They are hard to photograph, but I tried. Look especially at the blunt end:



If you hold the egg in front of a light source, the mystery is revealed - thin shell areas light up:



Your eggs simply have a thin shell. This may be caused by various reasons (older hens and lack of minerals amongst them), but is completely harmless as far as food safety and use in the kitchen is concerned.