Allergy reaction to silicone sheets? (Photo)

It looks like you have a great result following your breast reduction. Removal of the silicone sheets should be enough for this to resolve on its own. The yellow is bruising and is normal. I have used mild steroid cream for redness that does not settle although I would run this past your PS before self treating.

Silicone sheets on surgical scars are very helpful to manage scarring, are very easy to use, and work very well to minimize scar appearance.  
However one of the most frequent problems with the sheets are redness and itching under the sheets.  This is not an allergic reaction.  It is more than likely "fungal excoriation" (diaper rash, athlete's foot, jock itch) caused by trapping moisture close to the skin and allowing yeast to proliferate and irritate the skin.  Common symptoms are redness, itching and weeping inflamed tender skin, directly under where the sheets lie.  It can mimic bacterial skin infection so it is important to be seen by your surgeon, however skin infection (cellulitis) is rare and is almost always accompanied by fever and malaise (feeling like crap).  The solution is to take a break from the sheets and let the skin dry out.  Anti-fungal cream can help the symptoms and itching resolve faster. Steroid creams can help initially but actually promote yeast and will prolong your ordeal.  You can resume using the sheets after you become more comfortable (1 week), but limit use to 8-12 hours per day.  Hope this helps and good luck.