private void fillSouth(int x, int y, int fill, int old) {
    if (y >= raster.height) return;
    if (raster.getPixel(x, y) == old) {
        raster.setPixel(fill, x, y);
        fillEast(x+1, y, fill, old);
        fillSouth(x, y+1, fill, old);
        fillWest(x-1, y, fill, old);
    }
}
	

You can figure out the other routines yourself.