Making background transparent using gifsicle

I was recently turned onto gifsicle by another question, for making backgrounds transparent. Currently I am working on this gif file: . This purple background I'd like to make transparent. Currently I'm using this code:

gifsicle -U --disposal=previous --transparent="#fefffc" -O2 "C:\...\Tamibarotene binding.gif" > "C:\...\Tamibarotene binding trans.gif"

Where #fefffc is the code for this purple colour. Likewise "..." is the path to said file. With no visually-apparent results of this code, in terms of the appearance of the output file.