An old Celtic tradition, which is a a deeply moving tradition of binding of two people, it's where we get the phrase, "Tying the Knot".
You can choose to use as many colours as you like based on their symbolism.
White - purity, concentration, meditation, peace
Black - strength, wisdom, vision, success
Brown - earth, grounding, talent, telepathy, home
Green - finances, fertility, charity, prosperity, health
Yellow - charm, confidence, joy, balance
Red - passion, strength, lust, fertility