Answer
The color difference reinforces the species difference in areas of sympatry, which might otherwise be confused by pollinators, leading to wasteful hybridization.
Work Step by Step
Think about how the flower color is similar between the species in the allopatric case but not the sympatric case. This points to the need to distinguish the flowers better with sympatry--in this case, so that character displacement leads to different pollinators using the different species' flowers, avoiding hybridization.
