Vermillion
Usage notes
Some authorities view this as a misspelling; others consider it a valid but perhaps dated alternative. The OED records 23 citations with this spelling, ranging in date from 1597 to 1865.
About 30% of the usage of vermilion/vermillion in the US (COCA) is in this spelling. In modern British usage, this spelling is rare (only 2 publications in the BNC, out of thousands).
Only the one-L spelling appears in Webster's 1828, 1841 and 1913 dictionaries.