To make a colour coded section in your book, brochure, magazine or guide put a colour-filled bled object over the outside edge of each page of the section.
When printers cut pages to format the colour on the edge will be visible even when the publication is closed. Multiple pages with the same object will form a coloured section in the publication.
The picture is a detail of a Directory section in the Guide to Buying Property in Normandy I made two years ago. I created a long rectangular shape, filled it with yellow and dragged over the outside edge of the page. The same shape is on every page of the Directory in the same position on the outside edge.
One word of caution: don't overdo coloured edges. Too many small sections may be confusing for readers. And make sure that colours have some connection to contents, e.g. yellow – for directories as in Yellow Pages, green – for something ecological, pink – for romantic stuff like personal ads or dating section, etc.
See the photo of the printed brochure with a coloured section in this post.