That's right -- plain old Vaseline! Otherwise known as petroleum jelly.
I'll start with the finished journal spread first.
Yes, that's a young Winston Churchill on the right, along with a lovely lady (his mother, I think). He looks bemused by the bits of handwritten letter to his right. (The excerpt in the middle says "Don't think there's any danger of his company." I like that.)
This spread started with newspaper, torn apart and glued down using matte medium.
Next I smeared Vaseline over areas I wanted to keep unpainted. Then I covered the spread with green paint. After it dried, I wiped off the Vaseline.
I repeated this step with yellow green and dark blue. I ended up with this background.
At John's suggestion, I added a bit more red to bring out the bits of red newspaper still visible.
I liked this technique -- it's easy and the results are dramatic. Plus the Vaseline is non-toxic (unlike some art supplies) and makes my hands soft!