Blue is one of the three primary colours, the eye perceives blue when observing light with a dominant wavelength between approximately 450 and 495 nanometres.
Blue is the colour most commonly associated with harmony, faithfulness, confidence, distance, infinity, the imagination, cold, and occasionally with sadness.
When you visit a place like Polignano a Mare, where the windows are the same color of sea, you can only feel a sense of harmony.