I have thoughts. Fewer and farther between the last few years, but... OH!
Painting your fuselage tubing or engine mount (or race car frame) is an EXCELLANT idea, because steel, when cracked, exudes "smoke" when the crack "moves around". White paint makes it a snap to see that action, and you have to prevent rusting somehow anyway, so why NOT white?.
But wood spars are a different matter. They are left in CLEAR varnish so that you may visually spot "blackening" that indicates rotting and because varnish is the best sealant. Also wood doesn't produce the "smoke" at the crack location either, so there is no reason to think it would "aid in inspection". I know if I looked in a wing on an airplane I was thiinking about buying and saw anything but clear varnish (poly spar varnish is ok), I'd be putting my flashlight away and walking away from it.