One-sided etymology
There is a feeling that idioms resist interference. A red herring cannot change its color any more than the leopard can change its spots. And yet variation here is common. For instance, talk a blue streak coexists with swear (curse) a blue streak. One even finds to swear like blue blazes (only the color remains intact). A drop in the bucket means the same as a drop in the ocean. We can cut something to bits or to pieces, and so forth.