Replace colors in colormap

I use a simple line to break an indexed image 256 color into palette using

import numpy as np
from PIL import Image

im = Image.open('')
palette = np.array(im.getpalette(),dtype=np.uint8).reshape((256,3))

#####################
Printed result
[[  1   3   0]
[  2   4   1]
 [ 28   0   4]
 [ 20   2  26]
 [ 24   5  18]
 [ 33   7  22]
 [ 36   7  12]
 [  0  20  18]
 [ 42  15  16]
 [ 43  18  30]

... etc

Printing 'palette' lists the colors as RGB values as listed from index 0 onward. Index 0 is often dark color or black. In some engines it is used for alpha, transparency. I want to use commonly used colors for transparency like Magenta 255 0 255

I want to take each of my png files in a folder and do batch (I will have to manually add the color to the images, then save them as 8 bit so the color is part of the palette) then do:

exchange position of index 0 color with the Magenta in the color map
position of magenta color will vary for each file, just find the color 255 0 25 and replace color at index 0 with it but also take the index 0 color and put it on magenta place
do for all .png files in folder with a single run (magenta will be added and images indexed before running the script)

Solution

I think you want something like this:

#!/usr/bin/env python3

import numpy as np
from PIL import Image

# Open image
im = Image.open('image.png')

# Extract palette
palette = np.array(im.getpalette(),dtype=np.uint8).reshape((256,3))

# Look through palette
for index,entry in enumerate(palette): 
    # Swap this entry with entry 0 if this is magenta
    if index>0 and np.all(entry==[255,0,255]): 
        print(f'DEBUG: Swapping entry {index} with entry 0') 
        palette[0], palette[index] = palette[index], palette[0]
        break
else:
    print('ERROR: Did not find magenta entry in palette')

# Replace palette with new one and save    
im.putpalette(palette)
im.save('result.png')

You would probably code it to accept multiple files on the command line like this:

for file in sys.argv[1:]:
    ...
    ...

Then you could run:

UpdatePalette.py *.png