Thank you for sharing your photograph and questions. It appears that you have lost some tissue from the upper abdominal skin flap used to close your wound and it has been replaced with the yellowish tissue in your picture. Though there does not appear to be an infection taking place your body will have to heal this from the inside-out over the next several weeks. This type of wound often appears worse before it starts to improve so maintain close follow up with your surgeon to ensure an uncomplicated remaining recovery.
This is something that sometimes happens in the middle of a tummy tuck incision.As long as it is kept clean this will heal fine.Juist be cautious of excessive reddness and pain which might mean infection.
this is necrosis. The small area that is whitish is dead skin that did not heal. My suggestion is to have the surgeon take a look and he or she may need to clean that off. You will have a small area of necrosis and this should heal but will take longer. I do not think it is an active infection but I would also start you on an antibiotic temporarily while this heals. Tummy tuck is a lot of surgery and you can get areas like these that just don't heal like the other areas. but it will. Be patient. Stay close to your surgeon.
Based on your photos you may have an infection. See you surgeon as soon as possible for an evaluation.