You can use the "Blues_r" colormap in the matplotlib library, which interpolates from blue through white to red. To use this colormap, you can modify the code as follows:

plt.imshow(x_train[0], cmap='Blues_r', interpolation = 'nearest')

The _r in "Blues_r" indicates that the colormap is reversed, which means that it goes from blue to red, instead of from red to blue. The interpolation argument is used to specify the interpolation method for resampling the image data. The value 'nearest' indicates that the nearest-neighbor interpolation method should be used.
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