Referring to a Gene Vincent song, what does "Well, she's the girl in the red blue jeans" mean?

The term "blue jeans" was generic and could refer to any denim trousers, and could, therefore, be any color. Today we would merely say "red jeans".

Red Blue Jeans are just that...

https://poshmark.com/listing/Red-Blue-denim-jeans-5f01459181a36caf62f4e4e4

The term "blue jeans" (now more commonly shortened to jeans, especially with the advent of other colors) referred to a style of pants patented by Levi Strauss in 1873. They were originally blue (and the dye was not colorfast - you often had blue legs after wearing them the first time if you hadn't set the color properly!), made from denim, and featured rivets on the pockets. The 1950s saw them become a symbol of youth rebellion, especially after they were worn by James Dean in 'Rebel Without a Cause'. during the 1960s they became standard fashion wear, and for many of us who were teenagers then, jeans are still a standard leisure wear. Once they became mainstream fashion, different colors were produced, and the word blue became less commonly used to describe them.

https://en.wikipedia.org/wiki/Jeans

Gene Vincent recorded 'Be-Bop-a-Lula' in 1956, when the item of clothing was called blue jeans. Someone who made the effort to have them in red (probably by home dying) was certainly making a fashion statement, and was someone who stood out from the crowd.

https://en.wikipedia.org/wiki/Be-Bop-a-Lula