You have to use view formatting to customize entire row in a SharePoint list.

If you want to just apply the colors to entire row, you should use additionalRowClass
property to format the view.
Example: Considering internal name of your lookup column is LookUpColumn
, you can use JSON like:
{
"$schema": "https://developer.microsoft.com/json-schemas/sp/view-formatting.schema.json",
"additionalRowClass": "=if([$LookUpColumn.lookupValue] > 75, 'sp-field-severity--good', if([$LookUpColumn.lookupValue] < 75 && [$LookUpColumn.lookupValue] > 50, 'sp-field-severity--low', if([$LookUpColumn.lookupValue] < 50 && [$LookUpColumn.lookupValue] > 25,'sp-field-severity--warning', if([$LookUpColumn.lookupValue] < 25,'sp-field-severity--blocked', ''))))"
}
You can get the correct internal name of your column by following this article: How to find the Internal name of columns in SharePoint Online?
Similarly, you can apply any of the predefined classes given in below links as per your requirements:
- Predefined classes
- Reusable SharePoint Online Classes for Modern UI
Update from comments:
If [$lkpMile.lookupValue]
is not working, try using [$lkpMile]
instead of [$lkpMile.lookupValue]
.
Not sure why [$lkpMile.lookupValue]
is not working for you. I have tried it my end & it is working fine for me.
Lookup column settings:

List view formatting output with simple JSON:

Microsoft documentation also says,
The lookup field object has the following properties (with example values):
{
"lookupId": "100",
"lookupValue": "North America",
}
Source: JSON formatting - Special string values - check "Lookup fields" section