…of colour for visual interest?

Take this off-camera flash picture, for example (taken with speedlites, of course):

Good, because it is using off-camera flash. But you might try to add some colour by using gels. I use the excellent HonlPhoto gels, part of the Honl Photo small flash modifier systems.

No, not like that.

But perhaps like this:

Much better, I think. And all that is needed is a simple gel on the background flash (ask me about the Honl Photo discount, by the way, if like me you are considering those flash modifiers).