Why did the traditional color of Santa Claus's suit change from green to red?

Santa was first depicted wearing red, by cartoonist Thomas Nast in 1863, but up until the 1950's Santa was normally depicted wearing a green suit, sometimes also brown and white.

His suit changed to red after Coca-Cola started running an ad campaign. The artist Haddon Sundblom, who painted him for this ad, dressed him in red to match Coca-Cola's color scheme.

After a short time, this became the popular color for Santa's suit and is these days the norm.