Want a tip? The lightest "value" of the vehicles should be darker than the background, or vice versa. (As long as they are not the same) Notice how your Z is mostly the same color as the background? I'd just toss a little value in the background and let the cars pop a little. A nice gradient would be fine; say, between the noses and lighter near the windshields. (Toss it in shop to do quick tinkering)
