An archive contains a great deal of paper.
You would discover paper made of different colours, and various thicknesses; paper made from linen, wood pulp or other material. (http://en.wikipedia.org/wiki/History_of_paper)
Modern paper undergoes some sort of bleaching or dyeing process to give it the colouring we recognize as we print a document.
This is a photograph of a sheet of modern day paper (not part of any archival collection.) This sheet of paper was placed on top of a light source and photographed. With this level of detail you can see some detail of the material used to make the paper.
On rare occasions, something goes wrong. In this case, This image is was cropped from the same image. Because a piece pulp was not bleached properly you can see even more detail what modern day paper is made of.