Symbolism, Imagery, Allegory
If Edward's attachment to Victoria's doll in Act 1 is a sign of his femininity, Cathy's painting in Act 2 is a sign of masculinity: both Cathy's and Lin's. At first, Lin tells Cathy to paint a princess. But when Cathy rejects that idea (and a few others), Lin suggests that Cathy "Paint a car crash and blood everywhere" (2.1.23). This sudden suggestion shows us that Lin isn't really invested in Cathy's ladylike behavior in the way that mothers in Act 1/ the Victorian Era would have been.
When Cathy finally does make a painting, we can't be totally sure what it is. Betty comes onto the stage and has a look at it, but sheltered Betty would never be able to accept something as violent as a car crash coming from the mind of a little girl. At first she thinks, "Well I think it's a house on fire. I think all that red is a fire […] Or do I see legs, is it a horse?" (2.1.159-161).
Betty's inability (or unwillingness) to see the red as a bunch of blood shows us that she can't quite stomach the idea that a little girl like Cathy would be having such violent thoughts. But those of us who know Churchill's work by now know exactly what's going on. Cathy is acting differently than the way traditional society expects her to, and traditional society (read: Betty) is refusing to acknowledge her difference.