FRANKFURT—Erika Torchia holds a glass of cloudy brownish liquid to the light in her dining room and considers one of Germany’s most popular soft drinks.

“It looks like carbonated swamp juice,” says the American.

It’s actually a mix of cola and citrus soda that Germans call spezi and gulp down in vast quantities. PepsiCo Inc. has made a version called Schwip Schwap since 1969. Coca-Cola Co. has marketed its blend, Mezzo Mix, since 1973. Scores of other brands are for sale.

Where some see a drink that resembles diluted cough syrup or rusty water—traces of sediment swirl in some bottles—Fritz-Kulturgüter, a boutique soda maker from Hamburg, sees a market opportunity.

“It’s not an appetizing color when people are not used to it,” concedes Mirco Wolf Wiegert, a Fritz co-founder, of his spezi, branded as Mischmasch. “It looks a bit strange, but we don’t care—it’s part of our strategy.” Fritz intends to expand in European markets such as Britain and France.