FILL2C fills the areas defined by a 2D data sequence X/Y and an optional threshold value with two user definable sets of colors
set 1 for Y <= TH (NEG)
set 2 for Y >= TH (POS)

X/Y NaNs/Infs (NI) are removed in accordance with PLOT.
a sequence of data that contains NIs is split into blocks of NEG/POS areas, which my be colored individually depending on the size of the color set(s).

see
> help fill2c
and the accompanying published M-file for help and a few examples for this self-explanatory snippet.