Colour guide for page 25 of NOW #3
Although the above image looks like a production page, colour guides are actually made up of two different pages. One is a re-sized xerox copy of the original art on a transparent overlay and the other is hand painted by a colourist. When the painted page is brought together with the overlay you are left with the finished article.
Here is what both pages look like when they are separated:
While colour guides are not as sought after as original art, the role they played in the production of a comic book is still an important one. In many cases they are actually harder to find than original artwork as they were often discarded unlike the original pages that were usually returned to the artist or sold to the publishing company.