Pleasantville

David (Maguire) and his twin sister Jennifer (Witherspoon) lead different high-school social lives. Jennifer is shallow and extroverted; David is introverted and spends most of his time watching television. One evening while their mother, Sherry (Jane Kaczmarek), is away, they fight over the TV. Jennifer wants to watch a concert on MTV, but David wants to watch a marathon of Pleasantville, a black and white 1950s sitcom about the idyllic Parker family, who live in the squeaky-clean suburb of Pleasantville, Iowa. During the fight, the remote control breaks, and the TV cannot be turned on manually.
A mysterious TV repairman (Knotts) shows up, quizzes David about Pleasantville, then gives him a strange remote control. The repairman leaves, and David and Jennifer resume fighting. However, they are then transported by the remote into the Parkers' black and white Pleasantville living room in 1958. David tries to reason with the repairman (with whom he communicates through the Parkers' television), but he succeeds only in chasing him away. With the remote no longer functioning, David and Jennifer must now pretend they are Bud and Mary Sue Parker, the son and daughter on the show.
David and Jennifer witness the wholesome nature of the town, such as a group of firemen who only rescue cats stuck in trees, as there are no fires to fight. David tells Jennifer they must stay in character and not disrupt the lives of the town's citizens, who do not notice any difference between Bud and Mary Sue, and David and Jennifer. To keep the show's plot Jennifer dates a boy from high school, but, due to her impatience, Jennifer skips all forms of storybook romance and has sex with himâa concept unknown to him and everyone else in town.
Slowly, Pleasantville begins changing from black and white to color, including flowers and the faces of people who have experienced bursts of emotion and personal transformation (usually through sex). Though against Jennifer's disruption at first, David eventually joins in when he begins to see the shows characters as real people and not just avatars. David introduces Mr. Johnson (Daniels), owner of the cheeseburger joint/soda shop where Bud works, to colorful modern art via a book from the library, sparking in him an interest in painting. Johnson and Betty Parker (Allen) fall in love, causing her to leave home, throwing George Parker (Macy), Bud and Mary Sue's father, into confusion. The only people who remain unchanged are the town's fathers, led by the mayor, Big Bob (Walsh in his final performance), who sees the changes eating at the values of Pleasantville. They resolve to do something about their increasingly independent wives and rebellious children.
As the townsfolk become more colorful, a ban on "colored" people is initiated in public venues. Eventually, a riot is touched off by a nude painting of Betty (painted by Johnson) on the window of Mr. Johnson's soda shop. The soda shop is destroyed, books are burned, and people who are "colored" are harassed in the street. As a reaction, the town fathers announce rules preventing people from visiting the library, playing loud music, or using paint other than black, white, or gray. In protest, David and Mr. Johnson paint a colorful mural on a brick wall, depicting their world, prompting their arrest. Brought to trial in front of the town, David and Mr. Johnson defend their actions, and their passion begins to influence the non-colored members of the population into becoming colored. David also eventually arouses enough anger and indignation in Big Bob that the mayor becomes colored as well. The mayor runs off, effectively ending the segregation.
With Pleasantville and its populace now in full color, Jennifer and David both reflect on how their actions in changing Pleasantville have changed their own views on how they perceive the real world and their place in it, noting that it had been far more difficult for them to become colorful as opposed to the other townsfolk, and the circumstances that eventually allowed them to. Jennifer stays to finish her education, citing that her poor academic record in the real world would not allow for college. David uses the remote control, now working again, to return to the real world, and surprises his mother with his newfound maturity. The mysterious TV repairman, who had been outside the house in his truck the whole time, smiles as he drives away.

Question: What touches off the riot?
Answer: A nude painting of Betty on the window of Mr. Johnson's soda shop.