Buddhist prayer flags, if you notice carefully are always in a set of five colors and in a particular order only. The colors makes more sense and easy to remember when the flags are vertical.
The order from top to bottom is blue, white, red, green, and yellow, representing the five elements.
Blue symbolizes the sky and space, white symbolizes the air and wind, red symbolizes fire, green symbolizes water, and yellow symbolizes earth.
This picture was taken in Sangla Kinner Camps, Himachal, India.