I've noticed that the Iconic tile becomes the accent color after the tile updates for a little bit or when you access the app through the tile. It turns back transparent after the next tile refresh, it seems.
-- Before --
-- After --
Here's the way I understand could be going on (someone correct me if I have the current understanding incorrect):
WP8 Update 3: Back button kills the app. Start button tombstones the app. Holding back and hitting X will kill the app.
WP 8.1: Back button is programmable per app. The app owner...