Go to settings, or photos, and choose a wallpaper, if the wallpaper is eligible for "the change" it will be reflected in the preview. As an example, I chose the "7" from WWDC, in the preview the text was black (now gray because of beta 2). I then chose a dynamic wallpaper and the text was white in the preview. That should help you. Sorry if this has been posted before. The pic has to have a huge amount of light colors to "qualify" for the change. A pic with the sun reflecting on an ocean will work but if there is a dock at the bottom of the pic the text color will stay white. Do you get what I'm saying.