I have never tried this with pony stains, but I used to work at a place that has white balanced lights. I brought in a pink haired Gingerbread as a desk decoration and despite there being no windows in the room, her pink stripe faded to white. (It went pitch black in a power outage. No outside light was getting in.) Unexpected and a bit disappointing, but it made me think about indoor sunfading!
I haven’t tried it yet, but if I did I would line a box with a reflective material like foil and place a white balanced lamp in a hole in the lid. I think I’d use a plastic box but that might not be essential. Then I’d place the pony inside (protected as if for regular sun fading) turn on the light and see what happened. I’d be very excited to see what results people get if they try this!