You need one of each colour you want to mix, Rob.
Such as Gold/Purple or Purple/Green. Depending on what two colours you want.
Take them to the Cooking Basin in Reiflem, and put both necklaces into it. You should end up with the double coloured necklaces.

It seems there's a bit of a problem with the Basin at the moment though. So it might be a good idea to wait a bit, until staff are able to get it sorted. If you try to combine two coloured necklaces, you might end up with a Multi-coloured Mardi Gras necklace, which isn't supposed to happen.

Hope this helps!