The regular Trello web app allows filtering by label via the "Search and Filter Cards" button in the sidebar. Is it possible to filter by label in either the native iPhone app or the mobile web app?
It's possible to filter in the Trello web app. Click the icon to the left of the board title, and select 'search and filter cards' form the menu. It will bring up the same menu as the one from the sidebar.
There isn't a way to filter in the native iPhone app.