zlacker

The pre-iOS 7 graphics have black text over a non-uniform background color as compared to white text over a uniform background color. This gives us ranges instead of a single value, but even in the worst case, black is a vastly more legible foreground color:

           | iOS 5-6     | iOS 7+ |
  ---------+-------------+--------+
  SMS      | 11.3 - 13.4 | 2.2    |
  iMessage | 11.8 - 14.1 | 3.8    |