This is likely related to GPS signal "quality". You may have locked onto X number of sats, but the signal quality overall is not good enough from them to activate "Safe to fly GPS" mode.
Try a different location or I have heard of a firmware issue with 1.4 that sometimes causes this. But you may not be on that version.
It may have gone green as you increased altitude, away from possible interference from nearby power lines?
Have you done anything or added anything at all to the top of your Phantom?