It appears that Kate used to wear these bangles on night outings to clubs.
The photo above was taken in February of 2007.
The bangles look to be mostly goldplated, but some have a silver tone to them. They might be two toned bracelets. It also looks like she forgot to hide her hair elastic band. 🙂
She also wore them on another evening trip around the same year.
I am fairly certain that her bangles are costume jewelry that came as a set. These are fairly common and can be found in many clothing chain stores.