Reference: pmInvert

pmInvert(pm) returns the pixmap pm with all colours inverted. Thus the pixel (r, g, b) is replaced by (1−r, 1−g, 1−b) in RGB coordinates. This maps, for instance,

white (1, 1, 1) -- black (0, 0, 0),
red (1, 0, 0) -- aqua (0, 1, 1),
green (0, 1, 0) -- fuchsia (1, 0, 1),
blue (0, 0, 1) -- yellow (1, 1, 0).