Here is an example for you:
{
"$schema": "https://developer.microsoft.com/json-schemas/sp/v2/column-formatting.schema.json",
"elmType": "div",
"debugMode": true,
"txtContent": "@LookUpColumn.lookupValue",
"style": {
"color": "=if([$LookUpColumn.lookupValue] == 'testA', '#ff0000', '')"
}
}
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