Great question! There are several colormap options built in to matplotlib. The one that best fits your description of interpolating from blue to white to red is "bwr".
Your code would then look like this:

```python
plt.imshow(x_train[0], cmap='bwr', interpolation = 'nearest')
```

For a complete list of color maps, you can take a look at https://matplotlib.org/stable/tutorials/colors/colormaps.html
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