Adobe ACROBAT 3D Manual page 713

Grayscale model
Grayscale uses tints of black to represent an object. Every grayscale object has a
brightness value ranging from 0% (white) to 100% (black). Images produced using black-
and-white or grayscale scanners are typically displayed in grayscale.
Grayscale also lets you convert color artwork to high-quality black-and-white artwork. All
color information in the original artwork is discarded; the gray levels (shades) of the
converted objects represent the luminosity of the original objects.
When you convert grayscale objects to RGB, the color values for each object are assigned
that object's previous gray value. You can also convert a grayscale object to a CMYK
object.