I read an rgb image
img = cv2.imread('image.jpg')
b,g,r = cv2.split(img)
Make blue and green channels of all zeroes, and merge these with your red channel. Then you will see just the red channel displayed in red.
Or you could just set the b and g channels of the image to 0.
img[:,:,0] = 0 img[:,:,1] = 0