sorel-estia-deactivated20160504
asked:
Here's a headcanon of mine: Cutiemarks glow when that pony is doing something in relation to what their cutiemark represents.

That’s a  good way of handling things, and it would glow more brightly the closer their actions are to the purest form of their talent.  Every day activities wouldn’t produce a very vibrant glow, for obvious reasons.