Negative is the simplest mask: all it does is invert the colors (black becomes white, red becomes cyan, et cetera).
MEKO (+ and -) and FL masks also invert the colors, so apparently-miscolored regions may not be immediately attributed to this mask. This mask is often used in conjunction with Rotate RGB, which also produces miscolored regions. If not used, it may still be confused for that mask.
Because each use of the mask returns the perfect opposite set of colors, a second application of the mask perfectly reverses it.