I create a LineRenderer in Unity 2018.2.1f1. I set the material to builtin Default-Line; which has Particles/Alpha Blended Premultiply. I set no shadow casting and no light probes. The rendered color is lighter than the color I gave:
After doing some tests in different projects and platforms, it looks like the reason to wrong color rendering is the Linear Color Space. I chose Linear Color Space for better light rendering.
But apparently even though there is no light in the scene, only Line Renderer’s color is not accurate.
I fixed the issue by creating new shader which converts a vertex color to linear space. You can see the shader on the image. I used 2021.1.0a6 Unity that is why Shader Graph is different a bit.