How to reproduce:
1. Open the attached 'project-1182330.zip' project
2. In the Hierarchy window select 'Terrain'
3. In the Inspector window select 'Paint Details'
4. In the 'Details' section select 'DeadGrass_2'
5. In the Scene window paint the grass on the terrain

Expected result: grass is colored green (same color as the prefab) in the terrain
Actual result: grass is colored red in the terrain

Reproducible with: 2018.4.7f1, 2019.2.5f1, 2019.3.0b3, 2020.1.0a4
Could not test with: 2017.4.32f1 (grass prefab breaks)

Notes:
1) Reproduced with the provided prefab only
2) The grass was added as 'Detail Mesh' in the 'Paint Details' sections of terrain