Steps to reproduce this issue:

1. Open new project
2. Go to preferences
3. Enable OSX Color Picker
4. Select 'Main Camera' for example
5. Click on background color in inspector to change it
6. Alter the color with OSX Color picker, notice it works fine
7. Create and open new script
8. Make a little change to it
9. Go back to Unity and wait for code to recompile
10. Try to change a color with OSX Color picker
11. It does not work anymore

You need to restart Unity for it to work again