The copper can take 700° F, while the Grey and Black are good for 500° F. That may be more than enough for a nitro engine, but it's your call. I havn't tried anything but the copper, and that's the one I see recommended the most.
Any of them should withstand the heat of a little nitro engine. Any of the silicone stuff withstands over 500° or so I believe. I used them all on my race engine and in my Mustangs. Never had any issues with any of them.