Apply conditional formatting to list column

mabel9915 This is not possible using SharePoint out of the box functionalities.

You will have to customize the list forms using Power Apps and then filter the drop down items using formulas.

Check below links for more information: 

1. Customize a Microsoft Lists or SharePoint form by using Power Apps 
2. Customize a form for a SharePoint list 
3. Create dependent drop-down lists in a canvas app 
4. Filtering a drop down from a Lookup table 
5. Drop down filter from lookup field 

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