Changing pixel color value in PIL

Here is the way I'd use PIL to do what you want:

from PIL import Image

imagePath = 'A:\ex1.jpg'
newImagePath = 'A:\ex2.jpg'
im = Image.open(imagePath)

def redOrBlack (im):
    newimdata = []
    redcolor = (255,0,0)
    blackcolor = (0,0,0)
    for color in im.getdata():
        if color == redcolor:
            newimdata.append( redcolor )
        else:
            newimdata.append( blackcolor )
    newim = Image.new(im.mode,im.size)
    newim.putdata(newimdata)
    return newim

redOrBlack(im).save(newImagePath)

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)

You could use img.putpixel:

im.putpixel((x, y), (255, 0, 0))