Python opencv4.1 RGB2YUV

import cv2 as cv
import numpy as np

matA=cv.imread("apple.jpg")

cv.imshow("apple",matA);

matB=cv.cvtColor(matA,cv.COLOR_BGR2GRAY)
cv.imshow("GRAY",matB)
matC=cv.cvtColor(matA,cv.COLOR_BGR2RGB)
cv.imshow("RGB",matC)

matD=cv.cvtColor(matA,cv.COLOR_BGR2YUV)
cv.imshow("YUV",matD)

matE=cv.cvtColor(matA,cv.COLOR_BGR2YCrCb)
cv.imshow("YCrCb",matE)