Which one has primacy in English: "vermilion" or "vermillion" (as a color) ? Both have entries in various dictionaries. Is the difference concerned with AmE/BrE ?
This is excluding proper names. If we include them, the picture doesn't change dramatically:
According to Webster's Third New International Dictionary of the English Language, vermilion or vermillion is derived from,
There may be something involved with the distinction of between l/ll in the derivation from ME/OF.
Interesting too that there are several cities in the United States that use both the l/ll spellings.
Here is an Ngram as well that clearly favors the single 'l' spelling: