Steps to reproduce:
1) Add new Graph widget to dashboard.
2) Open graph widget in edit form.
3) Set Base color as - "№%№"% (or empty) and press enter.
4) Error appears - "Invalid parameter "Data set/1/color": a hexadecimal color code (6 symbols) is expected."
5) Don't add Host and item to Data set, press Apply button.
Result_1:
Hexadecimal error disappear, and appears new error.

6) Now add Host to Data set (don't press apply on widget)
Result_2:
Hexadecimal error returned.

Gif with process:

Expected:
Consistent error. Maybe let it stay on the form whole time until correct color picked?