If a paint's page doesn't list any alternative paints, there are three usual reasons.
It says "No paints match these filters."
Your filters may be too narrow. For example, My paints only shows paints in your palette. Tap clear filters under Alternative paints to see every match.
You can also see this message with no filters on. That means no paint in the catalogue is a close enough colour match.
It says the paint has no colour data
If you see This paint has no colour data, so close matches can’t be calculated., we don't have colour information for that paint yet, so we can't compare it with others.
Why are some paints I think look similar missing?
Only close matches are listed, so paints that are a long way off are left out. A paint never appears as an alternative to itself, or to another paint with exactly the same name.
What can I do instead?
If you own paints in a similar colour, try Create a custom mix. It suggests a recipe from paints in your palette.