Apple Shake 4 - Manual (Page 651)

Chapter 23 Color Correction 651
Parameters
This node displays the following controls in the Parameters tab:
rExpr
Use this function to change the input red value, always represented by “x.”
gExpr
Use this function to change the input green value, always represented by “x.”
bExpr
Use this function to change the input blue value, always represented by “x.”
aExpr
Use this function to change the input alpha value, always represented by “x.”