154 Dark Red/New Dark Red / Dark Red
BrickLink: Dark Red
This color was introduced in 2001, with the 10019 Rebel Blockade Runner. Perhaps it was in the system even earlier, since a Harry Potter set and a Scala set from 2000 had cloth pieces in the color.
Dark Red was notoriously inconsistent. Around 2010 (when Lego started mixing their own colors), the name was changed to New Dark Red (with the same number). This reflected the change to a much more consistent color in both opacity and brightness/value.