To reproduce:
1. Create a new project
2. Open Unity > Preferences and make sure OS X Color Picker is disabled.
3. Open System Preferences > Accessibility > Display and increase the cursor size to max
4. Select the Main Camera from a scene
5. In inspector window press on a Background color to open the color picker
6. Select the Eyedropper tool and try to select any color from the screen

Expected result: the selected color is chosen
Actual result: color defaults to white no matter what color is selected

Reproduced on:
2018.3.0a10

Couldn't reproduce the issue on earlier versions because of this issue:
https://fogbugz.unity3d.com/f/cases/838702/