The ex-votos are folded around a post-card depicting nature.
The colors of the postcards relate to the colors of Fragonard. In both, the idealised nature has sour colors.
Postcards, in general, represent a perfect souvenir of nature: the one we want to keep with us when we are away.
Those ex-votos aim to protect the ideal of nature. But those sour unnatural colors remind us that there is a tension. They are saying "look at the nature you want to protect: the colors of its memory are already fading away".