Before the invention of the Pennifold Quaffle, dropped quaffles often fell to the ground. Red balls were easier to distinguish against the muddy, wintertime Quidditch pitch (QA6).
Quaffles are first colored red
Location in Canon QA6: Changes in Quidditch Since the Fourteenth Century
Type of Event Wizarding world
Ironically, soon afterward the modern Quaffle - technically named the Pennifold Quaffle after its inventor - was developed, making the colour change unnecessary: Daisy Pennifold had the idea of bewitching a Quaffle to slow its fall through air to about the speed of falling through water, so that Chasers had a better chance of catching the ball in mid-air rather than continually retrieving it from the ground (QA6).
From the Web
Pottermore feature: Quidditch collection
Harry Potter Wikia: Quaffle
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