Flat shading basic setup.
For a sanity check I created a fresh level and totally stripped down shaders. Yes I can use a multiply to remove the “base color roughness value” as I am going to call it.
If the base color multiply number is left on 1 it will effect the overall base color value as a note of caution. But if left at 0 - no light influence its shift is minimal.
Note: I should have made a screenshot of this as well, though it may be obvious, but base color with just a 3 constant input looks exactly the same as base color with a multiply 1.
