Materials using Unlit/Color shader are noticeably lighter when using Metal as the Graphics API compared to other APIs.

Steps to reproduce:
1) Open the latest attached project.
2) Build for macOS (or enable Editor Metal support on versions 5.6 and up).
3) Run the build.

A quad in the scene is using the Unlit/Color shader. When using OpenGL, the shader is dark grey. Switching to Metal makes it look light grey/white.
This occurs on the Editor when using Metal, on Mac standalone and iOS builds.

Reproduced on:
5.6.0b1, 5.5.0f3