Why is it that following DIEP flap for delayed breast reconstruction, the transplanted skin has a yellowed appearance?

I have been researching delayed DIEP flap surgeries and often notice in the photographs from afterwards, the transplanted skin often has a yellowed appearance. I understand that some skin color difference is to be expected from abdomen to chest but why yellow? Why not just a paler tone of your skin? Is this avoidable? Does it have anything to do with the health of the flap? Or poor technique by the surgeon? Thank you in advance for any help answering this question.