Unforunately the RGB input has a number of limiatations that can cause issues - though I'll double check the Light Ghost Gray color tomarrow (should be able to pick up atleast two additional samples here in town). That said...
RGB color gamuts come in a number of different flavors, and you also have the issue of jpeg files shifting slightly darker than the actual color that they are. You can see an example of this going on here: Article regarding scRGB Colors (scroll down to the side by side comparison of a JPEG and HD image). Assuming that the image is encoded properly (unless you are specifically trying to, it is unlikely that it is - not any fault on your part, these issues are not generally covered in scanner and camera manuals), depending on what RGB gamut the eyedropper tool is using in order to sample the colors - you can have issues there. Since the most readily available and free tools tend to be designed for HTML sampling, that is what I have based the RGB conversion formulas off from. More advanced software like Photoshop, PSP and even the GIMP allow you to configure that yourself...but most freebies like ColorCop are much more limited.
After all those issues are dealt with - you also need to get an eye for the shape and lighting of the subject. In your example picture, the top of the fuselage is right out...most of it is a hot spot so to speak. Along the same lines so is the bottom of the fuselage. The best place for sampling seems to be the area behind the "Rescue" decal on the lower half of that plate. When I sampled there, I got two matches - Lichtgrau and Light Ghost Grey.
Anywho, thanks for the feedback - and I hope that bit of explanation goes aways to explain some of the limiatations I am fighting against. I'll write up a better explanation on the RGB issues later tonight and send it off to be checked (I can tend to get a bit technical in explanations causing more questions than answers ).