I was half-expecting this thread to be about gardening
Seriously.... Of the Bouguereaus I've seen, it looked to me as if it were a basic raw and/or burnt umber (it seemed to vary a little) that showed through as underpainting. This is not to say that perhaps he tinted the foliage areas with a different tone... But - it was a long time ago that I saw a group of Bouguereaus at once (1990, Borghi Gallery, NYC), so my study of them was limited to my then 20-year-old brain.
What I know of greens in his palette is fairly accurate: Viridian (referred to also as Emeraude green), which he called "the coldest of greens"; Terra Verte, which he called "weak... but most tender in its quality," and, "a fine glazing color"; and Emerald green (or Veronese), "...there are places where one needs it, especially in sketching... it is well to use it." Chrome greens he puts down as "all bad."
Hope this info helps!