How to reproduce:
1. Open the attached project ("case_1187401-WebglIMGUIGammaIssue.zip")
2. Change the targetted platform to WebGL
3. Open the repro scene ("SampleScene")
4. Enter Play Mode and notice how the colors appear in the scene
5. Exit the Play Mode
6. Make a WebGL build and run it
7. Compare the WebGL build IMGUI to Editor IMGUI

Expected results: IMGUI colors in the build appear the same as in the Editor
Actual results: IMGUI colors in the WebGL build appear washed out

Reproducible with: 2018.1.0a2, 2018.4.10f1, 2019.2.8f1, 2019.3.0b5, 2020.1.0a7
Not reproducible with: 2017.4.32f1, 2018.1.0a1

Notes:
1. Tested with Chrome (Version 77.0.3865.90 (Official Build) (64-bit)), Firefox 69.0.1 (64-bit) Opera (63.0.3368.94)
2. The issue only reproduces when using Linear color space