Reproduction steps:

1. Open "Linear color space particles.zip" project
2. Click on "Particle System 5%" in the Hierarchy window
3. Click on "Start Color" in the Inspector window
4. See the Hex Color: #0C0C0CFF
5. Click on the Color Picker tool
6. Click on the Particle
7. See that the Hex Color is: #3B3B3BFF

Expected Result: The "Start Color" Hex Color should be the same as the output in the Particles
Actual Result: The "Start Color" Hex Color is different from the actual output color

Reproduced with: 2018.1.0a2, 2017.3.0b7, 2017.2.0p1, 2017.1.2p2, 5.6.4p1, 5.5.5p1 and 5.4.6f3 (#3D3D3DFF)

Note:
The is no lighting in the scene
Ambient lighting is set to Black