Why Mailboxes In Red Color?

Do you know why mailboxes are in red color? This question appears in mind since the day I received my first letter from my first pen pal. Because the letter, I started wondering is there any different design other than the one in my housing area which all mailboxes are made in iron and red in color. I was 10 years old. Since the question appears, I wish I can have a beautiful and unique one. But after that I found my answer of this silly question. We can actually get thousands of differences designs mailboxes in the market. Am I silly? :P