My parents' postcards

In my parents' kitchen they have a noticeboard full of postcards. It would seem that all their friends only holiday near lakes, the sea or ski slopes, anywhere that means that blue will the main colour on the postcard.

Is the international tourist industry sponsored by the colour?

The only postcard that is completly devoid of blue is the one sent them from Krakow. Obviously Poland didn't get the memo*


*This is a Good Thing!