Not sure what would cause the yellowing in the background.
I have the 7610 and it uses the same pigment ink. But i dont use the Paropy paper.
I use the jpss(which is stretchy) and 3g Opaque( which is Firm). So im not sure what would cause that color issue.
Mine come out just fine when printing and pressing with no yellow issue .When i print i choose plain paper but with photo quality.
When i press my images with Jpss etc i wait about 10 min before pressing ,so i know the ink is dry. And then i pre-press the shirt for 3-5 secs and then apply my design and press for 30 secs or so and peel it hot,stretch the shirt in a couple different ways and place parchment over the image and press for another 10 secs to make the transfer go deeper into the shirt.
Not sure if this link will help i found but it talks about yellowing of the paper.
https://www.photopaperdirect.com/blog/?p=1278
If you have any questions drop me a message.