Change specific RGB color pixels to another color, in image file

Question

I would like to change a single color with Python.

If a fast solution with PIL exists, I would prefer this solution.

At the moment, I use

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Answer 1

This is a modification of Joe Kington's answer above. The following is how to do this if your image contains an alpha channel as well.

import numpy as np
import Image

im = Image.open('fig1.png')
im = im.convert('RGBA')
data = np.array(im)

r1, g1, b1 = 0, 0, 0 # Original value
r2, g2, b2, a2 = 255, 255, 255, 255 # Value that we want to replace it with

red, green, blue, alpha = data[:,:,0], data[:,:,1], data[:,:,2], data[:,:,3]
mask = (red == r1) & (green == g1) & (blue == b1)
data[:,:,:4][mask] = [r2, g2, b2, a2]

im = Image.fromarray(data)
im.save('fig1_modified.png')

It took me a long time to figure out how to get it to work. I hope that it helps someone else.