Botched repair

I don’t really know what has caused the discoloration of this manuscript, and it’s hard to get a very clear idea even from the digital photograph, but it looks like an agent that was used to prevent further deterioration of the folios has reacted with the ink.

Livres 6-8 du Code théodosien. [Lyon].
Source: gallica.bnf.fr

Bibliothèque nationale de France, Département des manuscrits, Latin 9643, fol. 6r.