the onces that i have worked with also had the old dark amber colored residue, but in my experience that is what really old "hide" glue looks like. BTW Elmers is not tecnically a "hide" glue, it is a synthetic version but it makes a really good substitute for those who do not support melting animals into products.
Some Info:
http://en.wikipedia.org/wiki/Animal_glue
Hide glues:
http://www.grainger.com/Grainger/TITEBOND-Glue-4EMP1
http://www.woodcraft.com/PRODUCT/200...GLUE-1-LB.ASPX
http://www.audubonstrings.com/Animal...g#.UbIZhed7-84