[Python] Converting image to grayscale

I want to convert any image to grayscale, but I don't understand the difference between these implementations.

image = cv2.imread('lenna.jpg')
gray = cv2.cvtColor(image, cv2.IMREAD_GRAYSCALE)

gray1 = rgb2gray(image)

gray2 = cv2.imread('lenna.jpg', cv2.IMREAD_GRAYSCALE)

image1 = Image.open('lenna.jpg', 'r')
gray3 = image1.convert('L')


When I plot them, I get them in blue scale, green scale, green scale and gray respectively. When I should use each one?

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