I purchased the Arduino Giga + DisplayShield bundle less than 1 month ago. This exact problem happened to me! More specifically it appears not just yellowing but the color palette is no longer full RGB. You can tell there is some sort of dithering going on. I have some pictures as examples (see below.)
I opened a ticket with Arduino tech support. I would love it if there was a fix I could do, because it was working perfectly until yesterday when I turn it on and clearly NOT perfect. For the price point of the board and shield I expect it to work flawlessly longer than 20-something days. I don't have a 2nd display or Giga to swap and test which one of the boards is actually malfunctioning. If this isn't fixable Arduino should refund money or swap the boards at no cost.