Until sdk-8.2.3 the background color used by WatchUi.Picker was black by default (at least on the devices I tested) and could be changed. Since sdk-8.3.0 it is white, and code to change it in onUpdate() doesn't have any effect anymore. A picker implementation which uses white text, now draws white text on white background.
The Picker sample that comes with the SDK demonstrates the problem.
The issue seems to only affect the simulator; on my fr955, the background is still black by default and can be changed.