opencv - plt.imshow shows color images for grayscale images in IPython

Question

I have the following RGB image imRGB

import cv2
import matplotlib.pyplot as plt

#Loading the RGB image
imRGB = cv2.imread('*path*')

imRGB.shape
>>(128L, 128L, 3L)

#plotting
plt.imshow(imRGB)

I convert this to a grayscale image imGray

imGray = cv2.cvtColor(imRGB, cv2.COLOR_BGR2GRAY)

imGray.shape
>>(128L, 128L)

#plotting
plt.imshow(imGray)

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Question: Why does the grayscale image imGray is shown in color?

Answer 1

imRGB was a 3D matrix (height x width x 3).
Now imGray is a 2D matrix in which there are no colors, only a "luminosity" value. So matplotlib applied a colormap by default.

Read this page and try to apply a grayscale colormap or a different colormap, check the results.

plt.imshow(imGray, cmap="gray")

Use this page as reference for colormaps. Some colormap could not work, if it occurs try another colormap.