The App ClassicPAN has an Exposure Control which seems to work so I got to thinking about making white. White is not an easy color to photograph without various controls, white balance and Exposure Control for example. I tried various ways with degrees of success but the easiest of all was unexpected. I pressed SAVE in an App called iDroste and almost immediately changed my mind and hit CANCEL. I expected it to just stop but rather it gave me a small bit of my image on top of a white "page." A bit of cropping and I have made white.
One use of a white image would be as background for edges and frames. Another would be to get a good look at various surface treatments such as textures or grunge. In fact as an increasing number of Apps have the ability to make and save Presets a white image would be an ideal starting point for making custom effects. I haven't done an extensive search of iPhone Apps yet but I know that some desktop Apps will accept files made elsewhere so the combinations could be quite exciting.
Here are a couple from Grungetastic and a few from some other frame making software.