On Friday, Stella at the bookshop told me about a children's book (Age 2 and up) named "Hello, Red Fox", Alladin Paperbacks, 1998 . It has a series of colorful animal/plant collages with tiny black dots in the middle on the left page, and a blank right page. The idea is to concentrate on the black dot for ten seconds and then look at the white page, and see the image in its complementary color. So the Red Fox of the title appears in green throughout the book.
I have a hard time seeing these images, be it on white or black background, so I tried it in the store, and the black dot helped. After a while, after several tries, I started to see a red fox, a green heart, and a violet butterfly.
The book was intended for Mom so she and the grandkids can have fun, but Ben and I are keeping it for a wee while.
Here are author Eric Carle's notes.