After years of convincing the nation's coffee drinkers that dark-roasted brews are the classiest thing to fill a mug or takeout cup, Starbucks, Peet's, and a new wave of high-end chains are rolling out the exact opposite: light-roasted coffee.

The target customers for the new style of coffee are people like Jackie Russell, a retired school administrator in Los Angeles. Ms. Russell's son Ted Russell is something of a coffee connoisseur, who has shared his passion for Peet's dark-roasted coffee with his mother. Only problem:...