Putting up a sample page is actually more for your benefit than theirs. If the art is complex, it will help weed out under-qualified people.
Whenever I make a post looking for help, I give more info right away such as a sample image, the production time-frame involved, number of pages, and pay range. This makes it a lot easier to find someone, I feel. It also shows that you are willing to take that little extra step and take the process more seriously and professionally. No offense.
Also, if you want people to color a sample page as part of the process, please be willing to pay at least a modest fee. It not only shows them that you respect their time and efforts, but it also shows that you are not just gathering up a bunch of different colorists to color all the different pages of the book for free (some really sick people do in fact do this).