Without being in front of GTS at the moment, here's how I do it.
If you take the car into the livery editor it should stay that blue. Pop a big square on the side where there are flats and curves (align to camera, set projection angle and depth to max), find a blue that is reasonably close and select it. Then go back into the selector and open the color picker. Adjust the color numbers by 1-3 at a time until you get it looking as close as you can.
Be sure to take note of whether the paint might be metallic or pearl first and use the outdoor background so you can see it in light and dark.