If your tensor doesn’t contain a color channel dimension the colormap is undefined and matplotlib will use its default one.
In case the tensor represents a real grayscale image, it would have a color channel size of 3 where all channels are equal.
This code shows the behavior:
x = torch.randint(0, 256, (100, 100, 1)).byte()
plt.imshow(x.numpy())

# create grayscale image
x = torch.cat((x, x, x), dim=2)
plt.imshow(x.numpy())

While single-channel images could be considered grayscale, matplotlib treats them as a matrix.