import cv2 as cv
import numpy as np

img = np.zeros((480,640,3),np.uint8)

b,g,r = cv.split(img)

b[10:100,10:100] = 255
g[10:100,10:100] = 255
r[10:100] = 255

img2 = cv.merge((b,g,r))
cv.imshow("img",img)
cv.imshow('b',b)
cv.imshow('g',g)
cv.imshow('img2',img2)
cv.waitKey(0)