Take a look at Changeling's career (from Wayfinder Guide for Eberron, which is still technically content of the UA).
From the PHB:
Change of appearance
As an action, you can transform your appearance or return to your natural form. You can not duplicate the appearance of a creature you have never seen, and you return to your natural form if you die.
You decide how you look, including your height, weight, facial features, the sound of your voice, color, hair length, sex and any other distinguishing feature. You can make yourself appear as a member of another race, even if none of your game statistics changes. Nor can you appear as a creature of a different size than yours, and your basic form stays the same; If you are bipedal, you can not use this feature to become a quadruped, for example. Your clothing and other equipment do not change in appearance, size or shape to fit your new shape, which requires you to have some additional outfits on hand to make the costume as convincing as possible.
Even for the most astute observers, their trick is usually indiscernible. If you arouse suspicion, or if a cautious creature suspects something is wrong, you have an advantage over any Charism (Scam) Check what you are doing to avoid detection.
If you want a cantrip, the Cantrip that comes to mind is a minor illusion, however this requires an action to launch and other action to end the illusion, which would not be exactly "a flash".