import matplotlib.pyplot as plt
import numpy as np
from matplotlib import colors

inner = np.zeros((4, 4, 4), dtype=bool)
inner[1:3, 1:3, 1:3] = 1
outer = np.ones((4, 4, 4), dtype=bool)
voxelarray = inner | outer

facecolors = np.empty(voxelarray.shape + (4,), dtype=object)
facecolors[outer] = colors.to_rgba("blue", alpha=0.5)
facecolors[inner] = colors.to_rgba("red")

fig = plt.figure()
ax = fig.add_subplot(projection="3d")
ax.voxels(voxelarray, facecolors=facecolors)
fig.savefig("nested_volumes.png")

outer_facecolors = np.empty(outer.shape + (4,), dtype=object)
outer_facecolors[outer] = colors.to_rgba("blue", alpha=0.5)
inner_facecolors = np.empty(inner.shape, dtype=object)
inner_facecolors[inner] = "red"

fig = plt.figure()
ax = fig.add_subplot(projection="3d")
ax.voxels(outer, facecolors=outer_facecolors)
ax.voxels(inner, facecolors=inner_facecolors)
ax.set_axis_off()
plt.tight_layout()
fig.savefig("nested_volumes.png", transparent=True)