Objects appear lighter when Metal support is used in the Editor.

Steps to reproduce:
1) Open the attached project.
2) Make sure Game view is visible.
3) In PlayerSettings, toggle 'Metal Editor Support*' on/off.

Notice that the color of the blue sphere changes - as the Editor starts using Metal API, the objects in the scene appear lighter.
There seems to be no difference between OpenGL and Metal in standalone builds.
Only objects (spheres in this case) are affected, the default blue background does not change its tone.

Reproduced on:
2018.1.0b2, 2017.1.0a6