How to equip the white winter-wonder-wasteland Pip-Boy?
Solution 1:
Found it. In the Armor Workbench, the Pip-Boy shows up as an inventory item. Select it, then choose Material and you'll be presented with a list of paint jobs that you've obtained.
Note: some people have found that certain mods prevent the Pip-Boy from appearing in the workbench inventory. If you don't see it, disable your mods one at a time until you figure out which one causes the problem.