Thanks, Nick. I don't know if it's possible to have more control over it. I know nothing about editing the shaders themselves, but I eventually worked out what controls the colour of bokeh, so that's what I'm fiddling with for these (along with the usual bokeh quality and rotation controls in the cfg file). It's very sensitive to movement, though, and will flip between opposite colours if I move the camera even a touch. It makes composition tricky if you want to get just the right colour. I don't know enough to understand if there's a way to counteract that. Still, with patience it's possible to get one or two decent shots.