Why are my residues/sidechains turning white? & what exactly does this mean? (btw I am using Foldit Standalone)
I'll assume standalone works somewhat like the online game, and that you're using the original interface, not the selection interface.
Coloring is controlled by the view options. From your example, I think you have the "Color mutated segments" option set. This options turns mutated segments white.
To check your view options, first, on the main menu (the one that says "Menu"), open "General Options", and make sure "Show advanced GUI" is checked. This gives the full set of options.
(The General Options menu is also available via control+T / command+T shortcut.)
Then, expand the View menu. At the upper right, you'll see several Color options, and below that, several View Protein options. These are what mainly control coloring.
You'll probably find the "Color mutated segments" option in the left-hand column, across from the "View Sidechains" menu.
In the selection interface, General Options is also available at control+T / command+T, or on the Main menu, which is most easily accessed via the lowercase P shortcut. The View menu is most easily accessed via the lowercase V shortcut. Clicking on the Foldit icon in the lower left or using the lowercase H shortcut reveals icons for the various menus, which are floating windows in the selection interface.