How can I control white balance? Every other digital camera I've used has a white balance control. I can't imagine a camera without one, since fundamentally the camera can't know what the "correct" white balance is without this human input.

Where is it in the iOS camera?
White balance on the iPhone using the default Camera app is set automatically so you can't change it during or before you take the picture.  

You can adjust it after the fact or you can use a 3rd party app that has white balance controls.