The pink ribbon has long been associated with the initiative for breast cancer awareness. This practice began in 1991 when the Susan G. Komen Foundation gave pink ribbons to the participants of an event that was being held for breast cancer. It was a race in New York City for breast cancer survivors. The rest, you can say, is history.
It is also worth noting that Charlotte Hayley, a breast cancer survivor, started her own trend using peach ribbons. This was part of her effort to send a message to the government that more funding should go towards breast cancer awareness/prevention. The color was changed later on to pink anyway.