Provide additional option for selecting color on grouped view

Problem/Motivation

Currently has two methods for selecting series color when a view is grouped: it can be based on the values of an entity reference field, or it can be based on a "color field" set on the referenced entities. While these are helpful, there are other scenarios, such as grouping on a "list (text)" field, where it would be helpful to have another way of selecting the preferred colors.

Steps to reproduce

Create a view with a chart format.
Group by a "list (text)" field.
Colors will default to the colors selected in your charts default settings.

Proposed resolution

It would be nice to have an additional method for selecting based on non-entity reference field values.

Remaining tasks

User interface changes

API changes

Data model changes