Incorrect Opaque Colors in UI Builder and when using ForceGamma

Project description:

A UIDocument contains 2 squares. One uses the default shader, and the other uses the a custom shader that mimics the default shader.

Steps to reproduce:

1. Open the provided project
2. Open the UI Document in the UI Builder

Actual results: 

The gray squares look different

Expected results: 

Both gray squares should look the same

Reproducible with versions: 

6000.3.0a5

b2c6724a5f5fa921b3eb127f7561fd1db5cbb225