JSON Conditional formatting for list view colum

Unless I've misintepreted what you want you don't need to tangle with JSON for this. As you are already using PowerApps for the form you just need to add an extra text input control that saves the text value back to the SharePoint column.

I've done a very simple example in the image below. The data in the 3rd dropdown, ddField, just looks up the number of a farmer's field, 1 - 7 from a separate list, but he wants to save the name of the field. So the number from the dropdown is saved to a text input box called dataFieldno where the default value is ddField.Selected.Value. 

We then add another text box called dataField that uses multiple ifs in the default value as follows:
If(dataFieldno.Text="1","Top Field",If(dataFieldno.Text="2","Upper Field",If(dataFieldno.Text="3","Mid Levels",
If(dataFieldno.Text="4","Home Field",If(dataFieldno.Text="5","Garden Field",If(dataFieldno.Text="6","Nicol Field","Low Field"))))))

Then in the datacard the Update box is set to dataField.Text. 

So although you are selecting a number from the dropdown, it is the text that gets saved back to the list.

I'm sure there are better/shorter ways of doing this but it was just a quick example. If you want any more info or the detailed steps just let me know.

Rob
Los Gallardos