In Tibetan Buddhism, different colors will have different meanings. When you have your wonderful trip in Tibet, you can see these colors everywhere, such as: blue, white, yellow, green, red. So, the Tibetan pray flag have five colors.
Blue means space. It is believed that anger can be transformed into wisdom when meditating on this color.
White means air. White can cut the delusion of ignorance and turn it into the wisdom of reality.
Yellow means earth. Yellow transforms pride into wisdom of sameness when visualized in meditation.
Green means water. Green transforms jealousy into the wisdom of accomplishment.
Red means fire. Red transforms the delusion of attachment into the wisdom of discernment.
For why Tibetan Buddhism has different meanings of different colors, please read: Meanings of Different Colors in Tibetan Buddhism.