import pylab as plt
import numpy as np

Z = np.vstack([np.zeros((1, 256)), np.zeros((1, 256,)), np.zeros((1, 256,))])
im = plt.imshow(Z, interpolation='none', aspect='auto')
plt.colorbar(im, orientation='horizontal')
plt.show()

import pylab as plt
import numpy as np

Z = np.vstack([np.zeros((1, 256)), np.zeros((1, 256,)), np.zeros((1, 256,))]).transpose()
Z = Z[None, ...]
im = plt.imshow(Z, interpolation='none', aspect='auto')
plt.colorbar(im, orientation='horizontal')
plt.show()

img = np.zeros((800, 500, 3), dtype=np.float32)

%pylab inline
import cv2
z = zeros([400, 400]) # 400X400 zero matrix
r, g, b = z, z, z
img = cv2.merge([r, g, b])
imshow(img)