The fourth lavender
Old lavender is a dark, grayish violet. Maerz and Paul's A Dictionary of Color of 1930 listed three main lavenders and then added old lavender as a darker gray one. The book dates it as a color name in English to 1924. The code #796878 comes from the ISCC-NBS list, which calls it grayish purple, as the Wikipedia article on lavender shows.
Far from the CSS lavender
The CSS keyword lavender is #E6E6FA, an almost white tint. Old lavender is much darker and grayer. Lavender gray is a lighter cousin from the same 1930 book.

