The M3 GTR Strassenversion was only available in Titan silver metallic. However, in the game, the factory color of the car is not even close to that. It looks more like a dark brown than a silver. It is noteworthy that this error is relevant not only for the FM, but also for the FH series.
Here’s a photo of the actual car for reference:
But until then, here’s how you can replicate Titan silver metallic on your own.
I used Two-Tone Polished paint.
Color 1:
Hue: 0.58
Saturation: 0.06
Brightness: 0.78
Color 2:
Hue: 0.58
Saturation: 0.05
Brightness: 0.90
Result:
It’s also worth noting that the rear wheels should have concave rims, unlike the front wheels. This is clearly visible in the photo of the real car.