Matplotlib Colormap showing Incorrect Color

You can answer it like that :

import numpy as np
import matplotlib as mpl
import matplotlib.pyplot as plt

# How to create an array filled with zeros
img = np.zeros([256,256])
colormap = np.zeros([256,3])

#image:
for i in range(256):
    img[:,i] = i #on all columns I have the same value
    
#color map:
for i in range(256):
    colormap[i,0] = 1
    colormap[i,1] = (i+1)/256
    colormap[i,2] = (i+1)/256
    
#display the thing:
colormap = mpl.colors.ListedColormap(colormap)
plt.figure(), plt.imshow(img, cmap = colormap)

almost like you did in here Colormap it’s not composed of correct color. You just need to write the second part of your code (from red to white) and do it in 256 moves instead of 128.