Specific values for Threshold colors in dashboard

[spinillos]

Some customers asked to have specific values for threshold color. They need to find in the frontend what are the values to set.

The field is a string, but show it as enum could be nice.

[K-Phoen]

Threshold color can be any color value. I know that you can use named colors, hexadecimal codes and probably even rgb() notation. An enum wouldn't make sense here I think.

As often, it would be useful if the schemas described that for us 🤷

[spinillos]

We cannot do anything here since color could be any hexadecimal value.

The only thing that we can do is to add the existing Grafana fixed values.