How to reproduce:
1. Create a new Unity project
2. Create a cube in a scene
3. Create a prefab from a newly created cube
4. Create new Material
5. Change materials color
6. Add newly created material to a cube
7. Observe both material and cube - colors are light
8. Reimport all assets
9. Observe both material and cube - colors are darker than before

Expected result: Colors should be the same before and after reimporting assets
Actual result: Thumbnails get darker after reimporting assets.

Reproduced with: 2019.4.31f1, 2019.4.20f1.
Didn't reproduce with: 2019.4.19f1, 2019.4.0f1.
Couldn't reproduce because of another issue (1368973): 2022.1, 2021.2, 2020.3.