Warnings are thrown due to Linear color space set as default when platform switched to Android, iOS, tvOS (Linear instead of Gamma) and prevents making build.

Steps to Reproduce:
1. Open the Unity editor > Create a new project.
2. Select 3D option from the Template drop-down menu > Create project.
3. Go to BuildSetting->PlayerSetting
4. Switch the platform to Android/iOS/tvOS.
5. Go to Other setting and observe that color space is set to Linear

Note:
1. The issue is that now building an empty project takes additional steps.
2. Additional user knowledge (since you need to manipulate settings, where before you didn't need to)

Working Fine :
2018.1.0b4, 2018.1.0b3, 2018.1.0b2, 2018.1.0b1, 2017.2.0f3, 2017.1.0f3, 2017.3.0f3

Reproduce On:
2018.2.0a4, 2018.2.0a3, 2018.2.0a2, 2018.1.0b11, 2018.1.0b10, 2018.1.0b9, 2018.1.0b8, 2018.1.0b7, 2018.1.0b6, 2018.1.0b5

Environment:
Mac (10.13, 10.12), Windows10, Windows7

Notes :
Appears always