pixsubst — Change color values used in a PIX(5) format image
pixsubst reads pix(5) format pixels on the standard input and converts all pixels matching {R_in G_in B_in} to {R_out G_out B_out}. The pixels' values are specified as integers from 0 to 255. If {R_in G_in B_in} is not specified, the first pixel in the input stream is used as the value to be changed.