Restoration is the act of adding foreign material to a comic book through certain techniques to return its appearance to an ideal or original state.
Restoring isn't new to the party 
Restoring paper material is nothing new; the practice has been around for a long time. Restoring items, in general, has been a way to preserve the longevity of the said item. But comic books seem to face much more scrutiny in restoration than, say, your grandmother's antique china cabinet.
Many fans view restoration poorly and would prefer to take a lower grade Universal blue label than a Restored purple label. This is due in part to mistrust when it comes to restoration, as well as purple label books being largely less valuable than their blue label counterparts. One collector, JD Yoders from the CGC Comics Fans Facebook group simply said:
Yes. Restored books are inherently less valuable. I'd rather have a beaten up original copy than a restored one
Does blue always beat purple?
This seems to be the general consensus with many collectors, who will happily take .5 blue label books than a restored any day of the week. Collector Scott D'Errico from the CGC Comics Fans Facebook group believes it does depend on the book;
It also depends on the rarity of the book itself. I knowingly purchased a restored copy of All Select #2 last year because there are only a total of 37 copies on the CGC cencus.
Scott would then go on to send the book to Kenny Sanderson to have the restoration removed, and instead opting to have the book conserved. But conservation is a topic for another day!
Mistrust with restoration stems largely from collectors buying raw books only to find out after grading that they are restored, as restoration can sometimes be hard to detect. Other times the restoration is crude, resulting in the book appearing worse than it would have unrestored.
Yet despite that, restoration can be a great and affordable way for many fans to get the books of their dreams. I personally owned a restored Pep Comics #22 as that was the only way I'd get my grail book. As stated earlier, purple label books typically go for less than blue. Depending on the book you could still make a profit off of it, even with a purple label.
Detective Comics #27 always commands high dollar amounts regardless of the grade. For example, a blue label 8.0 sold for $107,550 in 2010. Four years later a purple label (trimmed) sold for $137000. While this is a rather extreme example, it does highlight the price discrepancy but also shows purple keys can be more affordable. Detective Comics #27 is also a rare comic, so finding one in any grade or condition is worthwhile!
Many fans agreed that restoration on books newer than silver age will not fare well once it's time to resell. Golden Age books seem to have more leniency due to the scarcity of many of the books published, and a restored modern book? Why bother?
Conclusion
Overall, restored books do have a somewhat lukewarm reception to collectors. Perhaps you are a collector who sees the value in a well-loved book, or maybe you want a presentable copy of a Golden Age key. No matter what kind of books you collect, your collection should always be about what you love, no matter what the label is!
