You've proved conclusively that those Heller kits of classic French subjects can, in the right hands, be built into very fine models. They do repay a modeler's patience. I love the colors you've used. Tres bien fait, mon ami.
I use Tamiya TS paints decanted from the spray can for my airbrush and mix them to suit my taste. That's is what I would do in this instance if I were building this Audi. As I said, I see lots of red in that particular picture of that particular orange. I'd start with the TS-12 or maybe TS-56 (Brilliant Orange) and add TS-49 (Bright Red) until I got a match.
Scale Finishes also offers an Audi color called "Samoa Orange" that is probably suitable. I would just use Tamiya black and orange. The photographs you linked to don't capture the colors well. The orange looks like it has a lot of red in it.
Every update you post makes me happy. The engine bay is looking very good indeed. There's so much detail at that scale and all the little wires and tubes and hoses add to the overall effect. Keep it up.
A very cool project. I love seeing these cars built. The version at the link you provided is insane. I can't believe how hard some modelers work. I'm currently working on a Hasegawa Lancia Stratos, which is a great kit.