It was Graham who raised it as a possibility at high speeds, though I think we agree it isn’t directly causable by merely painting.

The oscillation of a control surface is bilateral, caused by a resonating dynamic ‘feed-back’ between varying aerodynamically and mechanically induced moments of inertia. This occurs at a specific harmonic frequency. Change the centre of mass relative to the pivot on one of the moving bodies and you change its fundamental frequency, such that you might get ‘constructive’ feedback causing oscillation, ie you hit a ‘harmonic’.

The vibration of a flap isn’t a bilateral oscillation as such, its a buffet. Moving the centre of mass of the flap might alter the buffet frequency, but as it’s a mechanically truncated, asymmetric and low-amplitude rattle not a smooth and symmetrical wave it is less likely to hit any kind of harmonic with the aerodynamic behaviour of the flying surface to which it is attached, however you paint it. I am struggling for an analogy, but when I get one I’ll write it 🙂

Is there any possibility the aim was to avoid any paint in hinge or actuator mechanisms?