In the beginning of "We Need To Talk About Kevin", people are celebrating a tomato festival.
In the middle of the movie, we can see Eva standing in front of tomato sauce cans.
What is the significance of tomatoes?
Blood. There are other scenes that are cues to blood such as where Eva is cleaning red paint off her house. A scene of her washing the paint of her hands is cut against the discovery of her daughter's pet in the waste disposal. Does she feel like she has blood on her hands for Kevin's crime, for being a bad mother?
The tomato festival in Bunol, Spain is in the book and it is from Eva's life before Kevin when she is free, relaxed and happy. The tomato festival is the film's opening shot and it's starts in close so it's not immediately clear what is happening - it strongly hints at the horrors to come - then as the camera pans out we see Eva completely happy, this is a counterpoint to how she feels when Kevin arrives.
In a interview, the director Lynn Ramsey explained how they weren't given permission to use the brand of soup they wanted (presumably Campbells) so they had to design their own label. But surrounding Eva with red and blood-like substances hints at her questions about her own guilt and how others see her as culpable for Kevin's crime.