Yeah that happened to half my dragon once.
Most likely its either of both of printing with bad ink and using bad photo paper.
In my case it was cheap photo paper, some1 explained to me that some chemical they use to make the paper can also react with the ink if subjected to UV radiation, and result in this discoloration.
You can try spraying them with fixatives that block UV radiation, but in my experience that just slows the process down.
Other than that I've rebuilt the models that showed those symptoms using better quality paper.
My Variations on a dragon pic shows the cut out discolored torso part in the center.
Most likely its either of both of printing with bad ink and using bad photo paper.
In my case it was cheap photo paper, some1 explained to me that some chemical they use to make the paper can also react with the ink if subjected to UV radiation, and result in this discoloration.
You can try spraying them with fixatives that block UV radiation, but in my experience that just slows the process down.
Other than that I've rebuilt the models that showed those symptoms using better quality paper.
My Variations on a dragon pic shows the cut out discolored torso part in the center.