Why setting U and V channels to 0 leads to displaying a Blue picture?

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What I did:

• I read a BGR image and convert it to YUV color space.

• Then I set its U and V channels to 0

• I display the image and I see it is all blue (as if I set R and B to 0, instead)

Questions:

• Why setting U and V values to 0 leads the the G and R channels of the picture set to 0 also ?
• Why the image is displayed in blue (BGR color space) instead of YUV color space ?

Code:

import cv2
import numpy as np

img=cv2.imread('rgb_pic.png')
yuv_img=cv2.cvtColor(img,cv2.COLOR_BGR2YUV)
cv2.imwrite('yuv_pic',yuv_img)

cv2.namedWindow('YUV',cv2.WINDOW_AUTOSIZE)
cv2.imshow('YUV',yuv_img) # Displayed image is YUV, that is ok
cv2.waitKey(0)
cv2.destroyAllWindows()

im=yuv_img # Copy the YUV image to an other one
for in in range(im.shape[0]):
    for j in range(im.shape[1]):
       im[i,j,1]=0 # Set U to 0
       im[i,j,2]=0 # Set V to 0
cv2.namedWindow('Result',cv2.WINDOW_AUTOSIZE)
cv2.imshow('Result',im) # Displayed image is totally BLUE :(
cv2.waitKey(0)
cv2.destroyAllWindows()

Results:

yuv_img displayed like this (that is ok for me):

im displayed like this (I do not understand why it is blue):

Regards, from BEGUERADJ