My kids thought I was saying “vanilla” folders because of their color 
which actually makes a lot more sense anyway. Where they invented in the Phillipines? 
How about “Manila” or (“manilla”) paper:
Manila folders were made out of Manila paper, which is made out of Manila hemp. Manila hemp is made out of fibers from the abacá plant, native to the Philippines.
I remember thinking they were “vanilla folders” when I was a kid, too…