the pink is not an indicator of a pattern or hidden gene. it's just from applied pressure. like if my napping on an embroidered pillow, when i wake up, there's a flower pattern imprinted on my pink cheek from the pressure i exerted on the pillow while sleeping.
Edit: also a black light still would not show any pattern on the whites of a Pied. the white is like an eraser and obliterator of pattern and nothing will show up. on the other hand, a black light may show pattern on the white of a BEL as it masks patterns vs. destroying it.