Dick Grayson, the first Robin, is widely known as not only Batman's first sidekick, but quite possibly the first sidekick in comics ever (and certainly one of the most popular and most enduring. First appearing on March 6th, 1940 in the pages of Detective Comics #38, Dick Grayson would be one of the very few sidekicks in both Marvel and DC Comics to actually "graduate" and move on from the role and eventually become Nightwing. In the process, he even left behind his old costume - which was good thinking on his part, seeing as the costume was outright stolen from a Flash Gordon comic that debuted years before the Boy Wonder.
Flash Gordon began life as a comic strip in 1934, and was created primarily to compete with the then-popular Buck Rogers character. Flash was an average Yale University student when he and his friend Dale Arden decide to travel through space on a rocked developed by Doctor Hans Zarkov to stop Earth from colliding with the planet Mongo. There the trio have many adventures and interact with notable characters including the dashing Prince Barin and the evil Ming the Merciless. The property was quite a hit at the time, spinning off into radio dramas and eventually a Flash Gordon movie with a theme song memorably written and performed by Queen.
In a 1937 Flash Gordon comic storyline entitled "Beast Men of Mongo", Flash and Dale make the journey to Prince Barin's kingdom. The story taking place in the Forest Kingdom, many members of Barin's forces wear green clothes modeled after medieval attire - but Dale's appearance seems awfully familiar to those who've committed Robin's original costume to memory. The base is green, the vest is red and the accent colors - belt, gloves, and cape - are yellow. If one were to remove the hat, one could easily mistake this panel as one from an early Batman comic at a glance.
In the interest of fairness, both Robin and Dale's costume here seems to be based on clothes worn by the fictional Robin Hood. As one of noble birth, Robin wouldn't think twice about wearing bright colors and standing out. Yet Flash Gordon was quite popular, and artists do tend to "borrow" liberally from other source material (like Captain Marvel in an infamous early series); perhaps this was simply unintentional, as printers could only reliably print certain colors at the time.
Today, Robin is one of the most popular sidekick characters in comic history while Flash Gordon has weaned in popularity considerably, especially from the heyday of the 1930s. Indeed, the Flash Gordon film was more of a tongue-in-cheek comedy than a straight-laced adaptation of the source material. Most readers are more familiar with Robin as part of the Batman mythos than the origin of his costume - and they're certainly not familiar with Flash Gordon or any associated characters.