Steps to Reproduce:
1. Create a new project
2. In Hierarchy, Right-click > Effects > Particle System
3. In Inspector, click on Start Color drop-down and change it to Gradient
4. Click on Start Color color to open the Gradient Editor window
5. Hover mouse over the gradient field

Expected Result: Gradient field is unchanged after hovering over it
Actual Result: Gradient background becomes gray after hovering over it

Reproduced with: 2021.2.0a13, 2021.2.0a14
Didn't reproduce with: 2021.2.0a12, 2021.1.4f1, 2020.3.5f1, 2019.4.24f1