This example helps to show how the isolated areas of a Venn diagram can be filled / coloured.
It was created as part of this answer on TeX StackExchange.
Other options for creating Venn diagrams with multiple areas shaded can be found in the Overleaf gallery via the Venn Diagrams tag.
This example is a LuaTeX-based plain TeX project which implements a LuaTeX callback function to show the components of a typeset paragraph. It is designed to accompany a blog post and, for simplicity, it is not a full-blown “parser”—for example, it is not recursive and ignores a number of node types. However, it offers a useful starting point for anyone wishing to explore callbacks in more detail.