The Greyscale Color node generates a float value. When connected to a Color input, the result is a greyscale color, with 0 being equivalent to black, and 1 to white.
Value - The output value.
In Modo, the Greyscale Color node has a hidden channel that can be added to schematic by right clicking the node > Add channel > Value. Accepts connections from Modo numeric channels. See "Connections from Modo Channels" for more details.