These are the only two images I could find showing the speed brakes deployed. My guess is you can either paint the insides of the brakes white or light gray. If I had my druthers though, I’d paint the insides of brakes insignia red.
They can be painted either the FS36081 that Stik mentioned, or they can be painted to match the adjacent camouflage pattern. Same goes for the flap coves on European 1 aircraft.
That info comes from Fairchild drawing 160D900006, December 1978.
Edit: Is your Academy 1/48 A-10 an A-10A or A-10C? If it’s Academy’s new A-10C, I don’t think there have been any A-10Cs painted in European 1 camo.
It is the Academy 1/48. Yeah, I know there aren’t any painted in the Euro 1 colors. But I decided I’m tired of painting various shades of grey US jets. I even bought the Tribute decals to change up the greys but I don’t find those to be a challenge. I may still change my mind and do grey…
I’m with you on the grey paint. I’m building that same Academy A-10C and making it their demo team aircraft with the SEA camo on it. Besides…you never know…maybe their next tribute aircraft will be done with the European 1, and with everything going on in Europe right now, they just might bring that color scheme back.