I have created this small tool for a friend of mine and myself, to allow us to quickly get the correct color to use for vinyl and livery in Forza.
You can attach any picture from your own computer, click anywhere on the picture to capture that pixel and convert the color to the non-standard color system which Forza is using.
Matte is still based on the base-color of the car, on this picture here, I captured the colors directly from the McLaren F1 website to convert those colors into Forza Horizon 5. So it is just the color, not the glossiness.