Tried again with red and green, trying to get solid colours on the printing plate, both positive and negative, by rolling harder and using thinner tracing paper. The alignment was a bit off again.
I also got a strange texture in the green background, a pattern of worm-like shapes which I couldn’t account for.
The ghost images here are quite attractive though. When I went to Granada and visited the Science museum, I saw this exhibit in a section on the human body which reminded me of the print. The ghost image seems to look through the body and show its delicate inner workings.