Stack Exchange network consists of 175 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.
In german we call it »Schnittverzierungen«. There are different kinds of them, the most common one is »Farbschnitt«, where all sides are colored.
Here are some examples:
You could speak of edge painting or fore-edge painting, or gilt-edged for the golden variant.
Required, but never shown