Last year I used a paint pen to touchup scratches in a slate gray metallic finish.  The touchup areas were much, much darker.  I tried air brushing the areas next used paint form the pens, thinned 1 to 1 and while the finish was much better, it was still too dark.  Is the paint mixed the same for all three applications? Or is there a difference between the spray can paint and the paint in the paint pens and the brush bottles? I have several airbrushes that provide fine control for this type of work, however I would most likely use my Paasche VL brush since I have all three needle sets(1,3,5) to adjust for spraying fine to heavy pigments.
  For air brushing the touchups, how should I order the paint?  All three of our cars are at least ten years old with metallic finishes that have surely faded somewhat. Should this be mentioned when ordering a specific paint match?
thanks, SP4149

The problem is the paint has faded in 10 years. Unfortunately, we cannot add "fade" to the paint mix. Paint fades completely differentl depending on where the vehicle has been located, the finish care through the years, etc. There is no difference in the paint in the spray can or small bottles at all. 

To get a perfect match, we can custom mix your paint by sending us a part. See https://www.paintscratch.com/products/custom.html for more details.

Also, metallic paints need to be blended and I would advise reading https://www.paintscratch.com/touch-up-paint-directions/blend.htm. Metallics can be sprayed lighter or darker depending on many factors explained in this link.

The 2oz. would be the size to order along with a 2oz. bottle of lacquer thinner for mixing the paint. Let me know how your project turns out.