See this wikibook: https://en.wikibooks.org/wiki/Python_Imaging_Library/Editing_Pixels
Modifying that code to fit your problem:
pixels = img.load() # create the pixel map
for i in range(img.size[0]): # for every pixel:
for j in range(img.size[1]):
if pixels[i,j] != (255, 0, 0):
# change to black if not red
pixels[i,j] = (0, 0 ,0)