Upgrading from 5.x to 6.x

Problem/Motivation

What is the recommended process to upgrade from 5.x to 6.x

Problem one:
Composer - if we run upgrade composer will then remove the the color module as it's no longer a dependency.

I thought I would then composer require drupal/color to get it back so we can uninstall post upgrade.

However on running drush updb I got the following:

Cannot redeclare resolve_color

Grepping the codebase it only seems to be defined in that one location.
I've wrapped it in a if !function_exists call - however now I don't see the color settings migrated properly when running the update.

Note this site has a child theme build using the script - with a dxpr_theme as the parent theme.

Steps to reproduce

Install dxpr_theme v5.x
create a sub theme
customise the color
run upgrade to 6.x
Observer loss of custom colors

Proposed resolution

Some upgrade notes with steps to migrate and/or an upgrade task.

Remaining tasks

User interface changes

API changes

Data model changes