On many editions of the Chumash, Talmud, and even many more-modern books, you look at the side of the book (opposite the spine) and there are random colored splotches printed on the pages. (For instance, the HaMaor edition of the Mikraot Gedolot Chumash has red and blue splotches.) Any idea why those are there?
Example:
It's just a bookbinding technique.
Read: COLOURING EDGES from this Bookbinding Website.
I am not certain but I suspect that it is simply a decorative practice. I believe I have seen it done on older, non-Jewish books and I assume that the practice has faded in favor of more economical/contemporary styles. Jews who buy seforim, on the other hand, are a little more inclined for "classic" styles and or more interested in a more distinguished graphic design (Goldleaf is not common among general books anymore but the rule among seforim).
Additionally, while I recall there being basis for leniency, opening and closing a book with words stamped on the side (such as the owner's name) is an old problem mentioned in the poskim and having such decoration (which lacks any coherent figure) decorates the side while preventing such stamping.
If you ever look at sfarim that are commonly opened to specific sections (like a siddur), you'll notice that there are black lines around those pages that are more commonly used (you could, for example, land almost exactly on the last page of Shacharis). When the pages are colored, you don't see those lines.
Could be just as a cheap alternative to gilt-edged pages, such as you find on expensive books (both Jewish and non-Jewish).
I have once heard that such technique was done because in the early days of book-binding, paper was very expensive and some books, including judaica, was printed on 'recycled' or scrap paper. This paper would be of random colors and element exposures. When stacked, the sides of the paper would be the colored splotches.
I have also had this question for some time and I was glad to see someone else asking it. From my research into book binding techniques it is obvious that it comes from various decoration techniques. This style seems to come from the Victorian era, but I am not sure how it got into the Jewish book printing business and seems to have stuck around longer. I assume that it is because seforim are valued as holy books and therefore receive a bit more decoration. However, I also heard that the practice of painting book edges originated with accountants wanting to ensure that pages were not stolen or removed from their financial records.
Reference: Glossary of Book Terms.
However, the techniques that I have found online which most closely resemble seforim are marbled edges or spotted edges. The former is much more ornate than seforim usually are and the latter is much less pronounced, being just speckles. The Jewish style seems to have developed its own approach and I would be interested to see what the technique is for applying this decoration.
Here is how they do marbled edges: Edge Marbling with Samuel Feinstein - YouTube.
I think this opens a whole range of topics in Jewish publishing. Thinking about ornate title pages, binding, edge spotting, page layout, etc...
I have always heard that these blotches make it to where one cannot see the "stains" that are left on the sides of pages from the oil on one's fingers.
