paint conversion charts

iPMS Stockholm has published their interpretation of Tamiya paints vs other manufacturers:

https://www.ipmsstockholm.se/home/urbans-colour-reference-charts-part-ii/urbans-colour-reference-charts-humbrol/

Keep in mind that exact matches are highly unlikely, more like "in the ballpark".  HTH

The iPhone app MODEL COLORS, available in the App Store, provides the capability to compare colors of many manufacturers. It is free. It also has standards; Fed, RAL, RLM, BSC, and perhaps ANA. There should be an similar Android app, don't know it. 

The tool allows comparison between a cited brand and your specific brand or with multiple brands, or with what you have in your stash.  You can specify the degree of similarity n the comparison. 

You enter the colors in your stash by picking colors off the list or by scanning the bar code.  The shopping list function alone is worth it

To RealG's comment about ballpark color comparisons,  I've often said if you were to put 6 modelers in a room and ask them about a specific color you will get at least a dozen different answers.  

I use the App Hobby Color Converter.

I've done quite a bit of research on this, and the short answer is as others have said - there are some reference sheets, but expect them to be "close enough."

That said, I've been using an iOS app called "iModelKit" that has been extremely helpful (as well as a horrible time sink!).  It requires the occasional manual intervention to check if there are updates on manufacturer's color databases, but overall it has a terrific mixing function and a very easy to use comparison function between manufacturers.  There are a LOT of manufacturers though, so pare your list down and/or sort it to make it easier to find your preferred manufacturers at the top of the comparison page.