Discuss and create unified way of setting colors in Ert and Everest

Ert uses `COLOR_INDEX`, this is to ensure consistent color assignment to an ensemble. Everest uses `PlotConfig.next_color()` as of now. This has lead to a subpar color assignment method in `ensemble_selection_widget` (https://github.com/equinor/ert/pull/13579).

We should take a look at how colors are assigned and see if we can simplify it/standardize it across Ert and Everest

Compare Ert's COLOR_INDEX with Everest's PlotConfig.next_color(), then inspect how ensemble_selection_widget uses color assignment and review the referenced pull request 13579. Done means agreeing on and implementing one consistent color-assignment approach across Ert and Everest, with the widget no longer relying on the subpar method described.

Written by the indexing model from the issue text.