The Learjet has COVERS not experimental parts. Covers are RED for visibility to ensure they are seen when installed.
The Gulfstream window is actually "Flight Test Orange" not RED and is used for external probes to my recollection.
Unairworthy parts are marked RED such as when unairworthy but on-ground useable landing gear or wheels are used for moving aircraft
Precisely my point, blended with a bit of my sense of humor.
Have you ever seen any "first" flight of a new aircraft painted in any particular color except perhaps green?