Blendshape changes object color on its own

I use blender to generate a very simple cube, assign it a base color and a blend shape that shrinks the cube in one dimension. After these, I save the cube as a gltf file. Then, in gltf viewer powered by three.js (e.g.,, dragging on the blend shape will change the “color” of the cube.

This seems odd. I’m pretty new to this space. Is this a bug or a feature?

I’m attaching the blender python script, the generated gltf file as well as a video clip for the color changing effect.

import bpy

cube =['Cube']

mat =[0]
mat.use_nodes = True
mat.node_tree.nodes['Principled BSDF'].inputs[0].default_value = (0.1, 0.2, 0.3, 1)

sk_basis = cube.shape_key_add(name='Basis')
sk_basis.interpolation = 'KEY_LINEAR'

sk = cube.shape_key_add(name='Key')
sk.interpolation = 'KEY_LINEAR'
for i in range(4):[i * 2 + 1].co.z = -1
for i in range(4):[i * 2].co.z = -1

gltf file: Dropbox - 1.glb - Simplify your life

video clip:

Found the solution: Blendshape changes object color on its own · Issue #283 · donmccurdy/three-gltf-viewer · GitHub