POST to endpoint and GET data back
Avoid making multiple calls to express server, you can return filtered data from the POST call. Use single AJAX POST call. As your calls are being made parallel there is no way to know if the make and color would be set when you make the GET request.
Use === to strict-compare values 
Array.filter()
returns a filtered array so you can remove the newList. Look into Destructing

readJSONFile('cars.json', (err, json) =&gt; {
    if (err) {
        res.status(400).json({
            error: err
        })
    }
    const items = json.cars
        .filter((item) =&gt;
            item.type.toLowerCase() === make || item.color.toLowerCase() === color
        );
    res.send(items);
});

Convert the callback functions to promise and chain for readability. Promisify