the inverse of a color $rgb(X,Y,Z) is $rgb($calc(255 - X),$calc(255 - Y),$calc(255 - Z)) or $calc($rgb(255,255,255) - $rgb(X,Y,Z)). or if you like, what color must be 'added onto' the given color to arrive at white
"The only excuse for making a useless script is that one admires it intensely" - Oscar Wilde