Re: A340-600 decals in 1:200
You'll be hard pressed to find 1/200 scale silk screen decals for an A340-600. You might be able to use the detail decals off of Daco's long out of print sheet for the A340-200 (Sabena colours) which came out in the early 2000's. You will have to rely on a custom drawn and printed sheet. Draw Decals has 1/144th A340-600 decals but they are his "Digital Silk" prints, which are over rated (my opinion). This observation is based on personal experience with the printer Greg uses and using the decal itself. They have a velvet like texture and are difficult to use on compound curves.
I'm not sure about the level of detail you're hoping to achieve, but in 1/200 scale placards and such are virtually impossible to read. On the actual a/c the type face is anywhere from 1/2" to 2" depending on what the placard is for. Sometimes less is more if that makes sense.
Re- corogard. You might be best at painting. A 50/50 mix of medium grey and silver is a good start. Weather and shade using 10/90 mix of smoke and thinner loaded in your airbrush. Some corogard sections are lighter, others darker. Consult your references as this will very from jet to jet... especially on Airbus.