Steps to reproduce:
1. Open user attached project
2. Open Terrain inspector and select Paint tool
3. Go to Player Settings
4. Switch between Linear and Gamma Color Space
5. Notice how both texture previews in inspector and project window are getting darker in Linear Color Space

Expected result: texture thumbnails in Project window and Terrain inspector should not become darker in Linear color space

Reproduced in: 2017.3.0a7, 2017.2.0f1, 2017.1.1p3, 2017.1.0p4
Only the Terrain inspector preview textures are affected: 2017.1.0p3, 5.6.3p4, 5.5.4p5, 5.5.0p4
Not reproduced in: 5.5.0p3, 5.4.6f1

Project window texture thumbnail darkening regression since: 2017.1.0p4
Terrain inspector texture thumbnail darkening regression since: 5.5.0p4