Match a value to the brand to the key color.

## Description

I would like one of the output colour to always match the key colour.

## Why do you need this feature?

When using brand colours

## Additional context

The issue does not name any files, tests, or entry points. First clarify which output should match the key color and how matching is determined when generating a palette, then locate the color-generation entry point and define tests for the expected brand-color behavior.

Written by the indexing model from the issue text.