What is the significance of the red band hindus wear around their wrist?

Green is the colour of fertility, Orange / Saffron is the colour of renunciation, White is the colour of widowhood and Red since it is associated with the rising sun is regarded as the colour of a new life. Moreover the planet that rules the marriage i.e. Mars is reddish in colour. Most north hindu brides wear red because it signifies fertility and prosperity. It's just a traditional color.