Boosting the Luminance of the Key Using Gain and Lift

You can remove grey from the key by increasing the gain and lowering the lift values in the 2D Histogram node in the schematic. Increase the gain to eliminate the light greys that may be in the white area of the matte, and decrease the lift to eliminate dark greys in the black area of the matte.

Note: Adjusting these values increases the contrast and may harden the edges of the matte.