Photochromic ink

Today I drew out a woodland scene with bear cubs playing and reaching out for the toy boats floating through the trees. I then painted leaves on the trees with blue photochromic ink so that they appear under direct sunlight (or in this case under my UV lamp because there is no sun outside at the moment!) and then fade away when the light goes.
Here's one section of my page first under the UV light and then once it's faded...