Thanks, BB, for pointing out what I couldn't see staring me in the face. Goodness knows where I got that idea from... it doesn't look anything like colour addition.
Back to the drawing board... using the distance settings this time.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)