Old (and even some new) wildlife guides used to use this format, with species descriptions printed on Bible paper and photos or other colour representations on colour plates. This seems to be true in other techincal texts where full-colour printing of the entire book would be too expensive.

Rather boringly when the plates are referred to collectively, the term "plate section" is used. More often the photos would be referred to as a "photo(graph) section", or (especially when reproduced from paintings) just "colour section".

Examples (from online sources):

• "The ... reference point in any field guide is the plate section"

  A Field Guide to Birds of the Gambia and Senegal, Barlow, Wacher & Disley. (via Google books)

• "For a colour version of this figure please see colour plate section"

  Radiotherapy in Practice - Brachytherapy, Hoskin & Coyle (via Google books)

(Tipped-in) (color) plate

Partial quote from Wikipedia:

In the book trade, a tipped-in page or, if it is an illustration, tipped-in plate or simply plate, is a page that is printed separately from the main text of the book, but attached to the book.[1]

A tipped-in page may be glued onto a regular page, or even bound along with the other pages. It is often printed on a different kind of paper, using a different printing process, and of a different format than a regular page.

[...]

Typical uses of tipped-in pages added by the publisher include:

• color illustrations, generally printed using a different process (e.g. intaglio or lithography) and on different paper
• an author's signature, signed on a blank or preprinted page, before the book is bound
• original photographic prints
• maps, often larger than the book format and folded to fit
• coupons or reply cards
• errata sheets, only produced after the printing run
• a short addendum
• a replacement for a missing, damaged, or incorrectly printed page

As implied above, more colloquially, they can be called simply color plates or just plates.

Those are the glossy pages.

Back when people used Sears catalogs as toilet paper, you'd hope to get a new one before you had to start using the glossy pages.

Its called a (colour) plate section. They are a multiple of 32 pages (sometimes 16). They will also only occur after a multiple of 32 pages – it is to do with how the book is printed and bound (one giant page is folded until you get 32 sides). Printing a whole book in colour is expensive, just doing one section in colour saves money.