Eggshells look solid, but they're actually slightly porous, so the developing chick inside can breathe. This quality can be exploited when dyeing easter eggs:

mottled ‘sweat’ easter egg —20130420N8 (nokia phone)

Often, of course, people do not want the eggs to “sweat”, as the emerging fluid pushes the dye to the side (much like putting salt crystals on a wet watercolor moves that pigment around). The way to avoid this is to make sure the egg and the room are same temperature.

From the same batch, this time in blue.

I quite like the effect, particularly when it shows something of how the egg developed, as with this blue egg: the pores seem to be more active around the middle.

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