Clarified best practices pertaining to Color Palette values

[nickylimjj]

Description

Clarified the name, slug and color values and defined the best practices to use them.

Fixes #14989

How has this been tested?

N/A

Screenshots

N/A

Types of changes

Documentation Update

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.

[afercia]

Thanks @nickylimjj ! Combined with the examples above, the new text perfectly clarifies the intended usage. I'd just suggest to try to address the case of dynamic values mentioned in the issue, which is what happens for example with Twenty Nineteen:

it may be good to include guidance around dynamic values (such as those sourced from a site option). For example, while not all dynamic values could be described programmatically, it may be advisable to provide meaningful names for at least the default values when applicable.