Awwww, don't just go for a ginger one! You may like him/her, but your piggies might not! Make sure they get on, before you bring the new piggy home. And like I say, what you like, may not be what your current piggies like! Besides, plenty of non ginger piggies that need homes too :-)
In my experience, adding a third piggy to a well bonded pair is the bonding that is most likely to go wrong because you are going to end up with a 2+1 constellation much more often that with a working trio.
Rather than upsetting your happy and stable pair, I would recommend thinking about a rehoming another bonded pair from a rescue; they know the characters and will only rehome well bonded couples, so you do not end up with the problem of having to find a solution for a piggy on its own a few weeks down the line.