That’s from ambient occlusion. I believe you can fix it by setting all of the lighting normals to the same direction (probably directly downwards would be best).
@OP:
Here is some code to paste into the command bar to create these settings:
-- Lighting Settings
local Lighting = game:GetService("Lighting")
Lighting.Brightness = 0
Lighting.Ambient = Color3.fromRGB(255, 255, 255)
Lighting.OutdoorAmbient = Color3.fromRGB(255, 255, 255)
Lighting.GlobalShadows = false
Lighting.Technology = Enum.Technology.Voxel -- Only works in cmd bar
-- Part Settings
local Workspace = game:GetService("Workspace")
for _, instance in ipairs(Workspace:GetDescendants()) do
if instance:IsA("BasePart") then
instance.CastShadow = false
instance.Material = Enum.Material.SmoothPlastic -- Optional
end
end