How to reproduce:
1. Create a new project
2. In the Hierarchy Window create a new particle system
3. In its Inspector enable "Color Over Lifetime" and click on the color field
4. Add 3 keys in total, one at the start, one at the middle and one at the end of the gradient
5. Change the first key's color to black and the last key's color to white
6. Change the middle key's color to green but don't confirm the change by pressing the ESC key on the keyboard
7. Observe the first key's color

Alternatively:
1. Open the user-submitted project ("UnityBugTest2.zip")
2. Click on the "BackgroundSettings.asset" and in It's Inspector click on the color field
3. Change the middle color key's color but don't confirm the change by pressing the ESC key on the keyboard
4. Observe the first color key's color

Expected result: the middle key's color change is cancelled and the first key is unaffected
Actual result: the first key's color is changed to that of the middle key's previous color. On some versions the middle key's color change is cancelled and on some the change is saved even when ESC is pressed.

Reproduced in: 2019.3.0a5, 2019.2.0b5, 2019.1.5f1, 2018.4.1f1, 2017.4.28f1
Versions where the middle key's color is not properly cancelled: 2019.1.5f1, 2018.4.1f1

Note: Reproduces on both Mac and Windows with the Unity Editor color picker but does not reproduce with the Mac OS color picker