To reproduce:
1. Open a new project
2. Import the ProBuilder using Package Manager or Asset Store, if on older versions
3. Set the Color Space to Linear in Player Settings
4. Open the ProBuilder window and toggle the Lighting in the Scene View

Expected result: The ProBuilder GUI looks the same whether the Scene lighting is on or off
Actual result: The ProBuilder GUI appears washed out when the Scene lighting is on and unlit when it's off

Reproduced with: 2018.1.0a2 2018.1.2f2, 2018.2.0b4
Not reproduced with: 2017.4.0f1, 2017.4.3f1
Regression started in: 2018.1.0a1

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Fixed in 3.0.9.