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img = cv2.cvtColor(img, cv2.COLOR_RGB2BGR) # built-in function
img[:,:,0], img[:,:,2] = img[:,:,2], img[:,:,0].copy() # alternative option
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import cv2
bgr_image=cv2.imread('image.jpg')
rgb_image=cv2.cvtColor(a, cv2.COLOR_BGR2RGB)