Steps to reproduce:

1) Download attached project "UnityBugreport_AlphaLinearLighting.zip" and open in Unity.
2) Open scene "scene".
Note: Go to Edit >> Project Settings >> Player >> >> Other Settings >> and check if Color Space* is Linear.
3) Look at Game tab. You will notice that logo image is slightly transparent.(see attached image "transparentWhenBlack.png".
4) In Hierarchy tab, click on Canvas >> Logo_Image.
5) Change Image color to white(255,255,255,255).
Note: Image is opaque(see attached image "opaqueWhenWhite.png").

Expected results: Image shouldn't be slightly transparent when color is set to (0,0,0,255)
Actual results: Image is slightly transparent.

Reproduced with: 5.2.4f1, 5.3.5p7, 5.4.0b25, 5.5.0a3.