Somewhere between Tamiya XF-53 and XF-65 would be a good place to start. You could try adding flat black or flat white to these colors a little at a time until you liked how it looks.
What I do is cut 1" x 1" square pieces of white paper and mix paints and spray onto the 1" squares and then paste them into a small notebook and write the exact mixture beside the square I airbrushed.
For instance here is a color I was trying to match a while back and I mixed colors with glass eye droppers to measure the amounts of paint. The lighting is not that great here in the photo but you get the idea.
