One of the most interesting things about gouache is its opacity, which allows you to paint white on dark ground, in this case, cardboard. I’ve always saved the last page of sketchbooks, but not done much with them. Now I know what to do!
One interesting note - I used pencil to draw the concentric circles on top of the gouache. It’s visible mainly in the white areas, but the flash on my camera really picked it up on the black areas as well. Image rotated 90 degrees clockwise for clarity, meant to be horizontal.