https://stackoverflow.com/questions/74743995/replace-cells-in-ndarray-with-specific-value-one-after-

import numpy as np

# Example foreground
fg = np.ones((160, 160, 4), dtype=np.uint8)
fg[10,10] = [0,0,0,0]
fg[20,20] = [0,0,0,1]
fg[30,30] = [0,0,1,0]
fg[40,40] = [0,1,0,0]
fg[50,50] = [1,0,0,0]
fg[60,10] = [0,0,0,0]
fg[10,70] = [0,0,0,0]

# Example background
bg = np.random.randint(0, 255, (1440, 1920, 4))

# Find coordinates of all pixels with (0,0,0,0)
fg_zero_pos = np.where((fg == (0,0,0,0)).all(axis=2))

# Count them
fg_zero_count = len(fg_zero_pos[0])

# Create random replacement coordinates for each element of
r = np.random.randint(0, bg.shape[0]-1, fg_zero_count)
c = np.random.randint(0, bg.shape[1]-1, fg_zero_count)
bg_rand_pos = (r,c)

# Perform replacement
fg[fg_zero_pos] = bg[bg_rand_pos]

print(fg[fg_zero_pos])