I think it would make sense for them to offer the 5S in the variety of colours the 5C will likely come in.
If they don't, someone who is fixed on having, let's say, a green phone, and really wants an iPhone, won't consider the (more profitable) 5S as an option - thus Apple cannibalising their own profits not just for price considerations, but on 'style' because the 5C has more options to offer that the 5S doesn't.
That being said, they've done well to keep it under wraps if their are several 5S colours coming... although the champagne 5S leak might be part a small part of something wider!