5.6 was a long time ago. We did a big change to how metric visualization works in 6.1, and it looks like legend indeed got lost somewhere along the way.
Starting from 7.10 Lens became generally available and we encourage users to create visualizations through Lens editor. Looks like the team already has color implementation on their radar (see issue. You can follow the linked issue for updates on this and possibly let the team know your requirements there.
It is unlikely there will be changes to the "old" Metrics editor in the meantime.
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