Together the colors yellow and blue represent the mixture of corruption (yellow) and innocence (blue). Both Sybil and Esmé have blonde hair, showing how they have been corrupted. Since Esmé's parents died, she has been forced to take leadership in raising her younger brother. Death is a common theme through out the Nine Stories, and is part of growing up and corruption. Furthermore, Sybil eats olives from her mothers martinis. Therefore their is small amounts of alcohol on the olives she eats, symbolizing how corruption begins lurking around kids in small doses when they are younger. Overall, Sybil and Esmé have been corrupted to different extents, but also have some innocence left to them (Sybil is still a naive, curious child and despite Esmé's situation she does show some childish behavior). 
While Esmé and Sybil are similar in their sense of innocence and corruption, Seymour and X differ. Even though both Seymour and X have clearly been affecting from serving in the war, their coping methods and end result are much different. On one hand, Seymour finds comfort in friendship with younger kids because of the innocence. Unfortunately, Seymors yearn for innocence eventually tips him over the edge. When looking at his own feet verses Sybils feet, he is able to reflect on himself. Sybils feet are pure, while his are corrupt from war. After his long mental battle over PTSD, Seymour kills himself. X on the other hand, deals with his PTSD quite differently. X shows signs of hope during a dark time. He thinks he will at some point beable to return to the man he was before the war. WIthout Esmé, X might not have been able to reach this conclusion. Seymour did not have this, for his wife was absent.