What is "invisible green"

Invisible \In*vis"i*ble\, a. [F. invisible, L. invisibilis. See In- not, and Visible.]

1. Incapable of being seen; not perceptible by vision; not visible. Specifically:

   1. Not visible due to an inherent property, such as lack of color; as, the invisible air; invisible ink;

   2. hidden from view; out of sight;

   3. Not perceptible due to lack of light;

   4. Too small or too distant to be perceived; as, people on the ground invisible at cruising altitude.

      To us invisible, or dimly seen In these thy lowest works.
      --Milton.

2. Hidden from the public; as, invisible transactions.

3. imperceptible to the mind; as, differences invisible to most observers.

   Invisible bird (Zo["o]l.), a small, shy singing bird ( Myadestes sibilons), of St. Vincent Islands.

   Invisible green, a very dark shade of green, approaching to black, and liable to be mistaken for it.

n. A very dark shade of green that is almost black.