Grimace was redesigned as a purple creature in 1972.
Grimace first appeared in McDonald's advertising as a much more mischievous character with multiple arms, associated with stealing milkshakes. The character was subsequently simplified into the large, purple, friendly figure familiar from later McDonaldland campaigns.
The redesign made Grimace easier to recognize and less threatening to young audiences. It also changed the character's role from a food-stealing antagonist into a companion of Ronald McDonald and the other McDonaldland figures.
People sometimes remember Grimace as having always looked like the later purple character, but the early version was substantially different. The 1972 redesign is therefore an important moment in McDonald's character branding, even though the character's name and basic advertising role continued across the change.