Freestyle line color from underlying material not working as expected I'm trying to color freestyle lines using the underlying material. It looks like it should be able to pick up the RGBAa diffuse and specular channels, and the documentation agrees. But I can't make it work, it seems to only pick up the workbench-related &quot;viewport color&quot;.
Is this supposed to work on 2.90?
https://docs.blender.org/manual/en/latest/render/freestyle/parameter_editor/line_style/modifiers/color/material.html
The material object has a property diffuse_color which is the viewport color. The color you want to link is the Base Color of the Principled BSDF node inside the node tree of that material. Sadly I don't know of a way to access it from the modifier, but you could use a script like this to automatically update the diffusion colors of all objects based on that node:
import bpy

for material in bpy.data.materials:
    if material.node_tree is None:
        continue
    principled = material.node_tree.nodes.get('Principled BSDF')
    if principled:
        material.diffuse_color = principled.inputs['Base Color'].default_value

It will consider the value inserted manually to the Base Color field, not one connected from some texture etc. And it makes sense to replace diffuse_color to line_color both in the code above and in the freestyle color modifier.