Supernatural Feature / Involuntary Shapechange / Nuisance Effect?

[CeeDub]

Salutations!

Suppose a character (let's call her Alice) looks absolutely normal (Appearance may or may not be above average), unless she uses certain abilities; then certain aspects of her appearance change involuntarily. Could be glowing or unnaturally colored eyes, a change in hair color, or any of the Supernatural Features listed in Basic.

Is this a Supernatural Feature with a -5% Mitigator (Not Using <Ability>)?
Is this an Involuntary Shapechange? Even with a purely cosmetic (0 CP difference from base form) and involuntary-only Alternate Form, I don't see a way to model this Advantage into a Disadvantage, but maybe I'm overlooking something.
Is this a Nuisance Effect tacked on to the Ability that causes the change in appearance?
Oh, I just remembered as I am typing this: Spell Signature would be a suitable Quirk (-1) or Disadvantage (-5).

Okay. Good talk. Thanks. ;)

But! What if it's not an Ability (something the player/PC can control) that triggers the change, but something that happens when Alice loses control, e.g. when her Bad Temper or other mental disadvantage kicks in?

[Prince Charon]

I generally use Nuisance Effect. For example (from a very nerfed Kryptonian template I'm working on):

Penetrating Vision (ESP, -10%; Blocked by lead, -10%; Nuisance Effect: Eyes (including pupil and sclera) appear golden during use, -5%; Uncontrollable, -10%) 2 [13]

Yes, his X-Ray Vision can spontaneously activate when he's under stress. (This actually fits the comics: Clark Kent tried to join the US Army during WWII, but failed the vision test by accidentally reading the chart on the wall of the other room.)

[Nereidalbel]

Temporary Disadvantage seems like a decent fit, too.