Dear Andrew,
Column Formatting can be used to color the column or the item rows based on the condition.
To auto-populate the values on a column such as Risk Rating, you can use the calculated column.
Create a choice type column Likelihood with four choices Highly Unlikely, Unlikely, Likely and Highly Likely.
Create a calculated column such as Risk Rating at your side via clicking on +Add column>See all column types>name the column as Risk Rating>select the calculated column type and add the formula as follows.
=IF(Likelihood="Highly Unlikely","Negligible",IF(Likelihood="Unlikely","Low",IF(Likelihood="Likely","Medium",IF(Likelihood="Highly likely","High",""))))
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Result:
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If you want to auto-color the background of the Risk Rating column, you can use the code as follows on Risk Rating.
{ "$schema": "https://developer.microsoft.com/json-schemas/sp/v2/column-formatting.schema.json","elmType": "div","debugMode": true,"txtContent": "@currentField","style": {"background-color": "=if([$Risk_x0020_Rating] == 'Negligible', '#4ed400', if([$Risk_x0020_Rating] == 'Low', '#00d4b4', if([$Risk_x0020_Rating] == 'Medium', '#f02bd2', if([$Risk_x0020_Rating] == 'High', '#d40004', ''))))"}****}
Result:
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Best Regards,
Sincerely
Cliff | Microsoft Community Moderator