Two issues found:

First!
Steps to reproduce:
1) Create dashboard with Graph widget.
2) Open graph widget and navigate to override tab.
3) Setup override and add option - base color.
4) Open color picker and set any color for this override option.
5) Now try to add base color option again and press Apply button.

Result:

Error message appears

Expected:
Or nothing happens, or no color displayed.

Second!
Steps to reproduce:
1) Create dashboard with Graph widget.
2) Open graph widget and navigate to override tab.
3) Add 2 overrides with option - base color.
4) Change override orders.
5) Remove Base color option from top override.
6) Add base color again.

Result:

Base color incorrectly placed.

Expected:
The base color on the same position.