Creative Effects > Keyer > Adjusting the Luminance of the Key > 
Boosting the Key's Luminance Using Gain and Lift
 
 
 

You can remove grey from the key by increasing the gain and lowering the lift values in the Matte menu. 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.

NoteAdjusting these values increases the contrast and may harden the edges of the matte.