If anyone else has this problem, it's because I had to set painting mode to "Image"
This post explains how to set it up:
In object mode, everything is working. But when I switch to Texture Paint mode, the model turns white - so I can't paint on it. In the Piero tutorial, Kent does not have the same problem. Any ideas what might be wrong? Thanks!
This person with the same problem said to switch to Cycles Render, but when I clicked the bar at the top to switch from "Blender render" to "Cycles render" nothing changed.