The art of propaganda consists precisely in being able to awaken the imagination of the public through an appeal to their feelings, in finding the appropriate psychological form that will arrest the attention..
Propaganda should pull attention.
The more extreme the reaction, the better.
Colors of the image:
Quoting The Designer’s Dictionary of Color
Red is radical. It’s extreme, representing passion, energy, violence, fire and anger. It is vibrant and creates contrast, demanding the viewers attention.
Yellow can be used to create a bold contrast with other colors.