There's an archival tape called JLAR which I've 
used for years on torn posters, comics and 
whatnot. It won't turn yellow, and if you cover 
old tape repairs with it completely, the tape 
under it won't turn yellow either. I'm not sure 
why though. *shrugs*

You can find it at Talas here:
http://nt.bnt.com/talas/menu.html?
keywords=jlar&x=9&y=19

Hope this helps.

KF