Try adding below JSON on Quantity column:
{
"$schema": "https://developer.microsoft.com/json-schemas/sp/v2/column-formatting.schema.json",
"elmType": "div",
"txtContent": "@currentField",
"style": {
"color": "white",
"background-color": "=if(@currentField < [$MinQty], 'red', if(@currentField >= [$MinQty], 'green', ''))"
}
}
Use correct internal name of Min Qty field in place of [$MinQty].
Documentation: SharePoint column formatting